Should the supplication for hearing thunder be repeated if the thunder is continuous?
Question

“Subhan alladhi yusabbih ur-ra‘du bi hamdihi wa’l-mala’ikatu min kheefatihi (Glory be to the One Whom the thunder glorifies and praises, and the angels [also glorify Him] out of fear and awe of Him.” Should we repeat this dhikr if the thunder is continuous, or is it sufficient to say it once only?
Praise be to Allah.Firstly:
There is no proven report from the Prophet (blessings and peace of Allah be upon him) that he used to say this dhikr when hearing thunder. Rather it was narrated with a sound isnad from ‘Abdullah ibn az-Zubayr (may Allah be pleased with him) that if he heard thunder, he would stop speaking and say: “Subhan alladhi yusabbih ur-ra‘du bi hamdihi wa’l-mala’ikatu min kheefatihi (Glory be to the One Whom the thunder glorifies and praises, and the angels [also glorify Him] out of fear and awe of Him.” Then he would say: This [thunder] is a stern warning to the people of the earth.
Narrated by al-Bukhari in al-Adab al-Mufrad (723) and Malik in al-Muwatta’ (3641). Its isnad was classed as sahih by an-Nawawi in al-Adhkar (235) and al-Albani in Sahih al-Adab al-Mufrad (556).
It was also narrated from others among the Sahabah.
An-Nawawi (may Allah have mercy on him) said: They narrated from Ibn ‘Abbas (may Allah be pleased with him) that he said: We were with ‘Umar on a journey and suddenly there was thunder, lightning and hail. Ka‘b said to us: Whoever says three times when he hears thunder “Subhan alladhi yusabbih ur-ra‘du bi hamdihi wa’l-mala’ikatu min kheefatihi (Glory be to the One Whom the thunder glorifies and praises, and the angels [also glorify Him] out of fear and awe of Him” will be protected from that thunder. We said it and we were protected."(Al-Adhkar by an-Nawawi  p. 181).
Ibn ‘Allan said: al-Hafiz said: This isnad is hasan and mawquf [meaning that it stops at the Sahabi]. Even though it was narrated from Ka‘b, Ibn ‘Abbas and ‘Umar approved of it, which indicates that it has a basis.  
End quote from al-Futuhat ar-Rabbaniyyah ‘ala al-Adhkar an-Nawawiyyah (4/286).
Hence it is recommended (mustahabb) to say this dhikr when hearing thunder.
It was narrated in a marfu‘ report from the Prophet (blessings and peace of Allah be upon him) that when he heard the sound of thunder and lightning, he would say: “Allahumma la taqtulna bi ghadabika wa la tuhlikna bi ‘adhabika, wa ‘afina qabla dhalika (O Allah, do not kill us with Your wrath and do not destroy us with Your punishment and protect us before that).”
Narrated by at-Tirmidhi (3450), who said: This is a gharib hadith; we only know it through this isnad. It was classed as da‘if (weak) by an-Nawawi in al-Adhkar (p. 181) and by al-Albani in Da‘if at-Tirmidhi (3694).
Even though this hadith is da‘if, as at-Tirmidhi himself said, when it comes to the hadiths that encourage virtuous deeds in general, and supplications (du‘a’s) in particular, the matter is flexible, and the scholars usually took a lenient approach towards their isnads.
In al-Mustadrak it was narrated that ‘Abd ar-Rahman ibn Mahdi said: If we narrated a report from the Prophet (blessings and peace of Allah be upon him) that speaks about what is halal and haram, or about rulings, we are strict in examining its isnad and its narrators. But if we narrate hadiths that speak of the virtue of various deeds, reward and punishment, permissible matters and supplications, we are lenient in examining their isnads.
Al-Mustadrak ‘ala as-Sahihayn (1/666).
Al-Qadi Abu Ya‘la (may Allah have mercy on him) narrated from Imam Ahmad that he said: When we narrate [reports] from the Messenger of Allah (blessings and peace of Allah be upon him) that speak of halal and haram, we are very stringent in examining the isnads. But when we narrate reports from the Prophet (blessings and peace of Allah be upon him) that speak of the virtue of various deeds and reports that do not affirm or abrogate a ruling, we are lenient in examining the isnads."(Tabaqat al-Hanabilah 1/425).
An-Nawawi (may Allah have mercy on him) said: We have previously noted that the scholars are agreed that one may act on a da‘if hadith that speaks of righteous deeds, but not matters of halal and haram."(Al-Majmu‘ Sharh al-Muhadhdhab  3/248).
Al-Hattab said: Even though this is da‘if, the scholars are agreed that it is permissible to act in accordance with a da‘if hadith that speaks of the virtue of a particular deed."(Mawahib al-Jalil  1/17).
Ibn Hajar al-Haytami (may Allah have mercy on him) said: The scholars are unanimously agreed that it is permissible to act in accordance with a da‘if hadith that speaks of the virtue of a deed, because if it turns out to be sahih, then the one who accepted it will have acted as is required, and if it does not turn out to be sahih, there will be no negative consequences for having acted upon it, such as making something haram halal and vice versa, and there will be no transgression against the rights of others."(Al-Fath al-Mubin bi Sharh al-Arba‘in  p. 109).
The claim that there is unanimous scholarly agreement on this issue is not accurate, as we have stated previously that the scholars differed in the conditions that they stipulated for acting in accordance with a da‘if hadith (question no. 44877 ).
Shaykh Ibn Baz (may Allah have mercy on him) said: When it comes to hadiths in which there is some weakness, it is either due to a narrator whose memory was poor, or a narrator who has committed some mischief, or other, similar reasons for which it was said that his hadith is da‘if. A da‘if hadith may be quoted by scholars in the context of encouraging people to do righteous deeds that are established and well-known in Islamic teachings, such as the da‘if hadiths that speak of giving charity, because charity is something that is well-known in Islamic teachings, there are verses of the Quran which highlight the importance of charity, and there are sahih hadiths which speak about it. So if there are some da‘if hadiths which speak about the virtue of charity, the scholars quote them. The same applies if there are da‘if hadiths which speak about the virtue of supererogatory (nafil) prayers, or about the virtue of tasbih (saying Subhan Allah (Glory be to Allah)) and tahlil (saying La ilaha illa Allah (there is no god worthy of worship except Allah)), and of dhikr. All these matters are established and well-known in Islamic teachings, so the scholars may be lenient regarding da‘if hadiths that speak of them, because the original deeds of which the da‘if hadiths speak are rooted in the Quran and Sunnah, and because acts of worship that are mentioned in da‘if hadiths are already well known from the Quran and Sunnah."(shaykh’s website).
Secondly:
With regard to the issue of repeating the dhikr mentioned in the question – “Subhan alladhi yusabbih ur-ra‘du bi hamdihi ...” –
It is clear from the action of Ka‘b and the approval of ‘Umar and Ibn ‘Abbas (may Allah be pleased with them all) that he used to say it three times.
And it is proven from the Prophet (blessings and peace of Allah be upon him) that it is recommended to repeat supplications three times. When he offered supplication, he would say it three times, and when he asked of Allah, he would ask three times.
Based on that, if this dhikr is repeated three times, that is good.
If he repeats it every time he hears thunder, what appears to be the case is that there is nothing wrong with that, as it is proven that the Prophet (blessings and peace of Allah be upon him) would sometimes repeat a supplication more than three times. It was narrated that Jarir ibn ‘Abdillah (may Allah be pleased with him) said: The Prophet (blessings and peace of Allah be upon him) blessed the horses of (the tribe of) Ahmas and their men five times. Al-Bukhari (4099).
Al-Hafiz Ibn Hajar (may Allah have mercy on him) said: This indicates that he used to repeat supplications an odd number of times, and he sometimes repeated them more than three times. This is an exception to the general meaning of the words of Anas, “When he offered supplication, he would repeat it three times.” That is to be understood as referring to what he usually did, and when he said it more than three times, there must have been a reason which dictated that he should say it more than three times. This is clear in the case of the tribe of Ahmas, because of their efforts in standing up to disbelief and supporting Islam, especially with regard to their own people to whom they belonged."(Fath al-Bari by Ibn Hajar  8/73-74).
So if there is a reason for repeating it, such as if the reason for saying the dhikr is repeated, then there is nothing wrong with repeating it.
And Allah knows best.