When to Say Allahumma Barik
Question

If I see something that I admire, am I required every time I see it to say “Allahumma barik (O Allah, bless it)”, or is it sufficient for me to say that the first time I see it? If I did not say it on one occasion, is there any sin on me?
Praise be to Allah.When to say Allahumma barik
The Prophet (blessings and peace of Allah be upon him) instructed the Muslim, if he sees something with his Muslim brothers that he admires, to pray for blessing (barakah ) for them. The Prophet (blessings and peace of Allah be upon him) said:
“If one of you sees something with his brother that he admires, let him pray for blessing (barakah) for him.” Narrated by Imam Malik in al-Muwatta (2/939), Imam Ahmad in al-Musnad (25/355), and Ibn Majah (3509)
Should you repeat the du’a?
The scholars differed as to whether this command means that this should be repeated or not.
According to Usul al-Fiqh, if there is a command to do something with no indication to suggest that it should be done repeatedly, then it does not have to be done repeatedly.
Shaykh Muhammad al-Amin ash-Shinqiti (may Allah have mercy on him) said: 
“With regard to the report narrated by Muslim in his Sahih from Abu Hurayrah, who said: The Messenger of Allah (blessings and peace of Allah be upon him) addressed us and said: “O people, Allah has made Hajj obligatory for you, so do Hajj.” A man said: Is it every year, O Messenger of Allah? The Prophet (blessings and peace of Allah be upon him) remained silent until the man said it three times. Then the Messenger of Allah (blessings and peace of Allah be upon him) said: “If I said yes, it would become obligatory and you would not be able to do it.” Then he said: “Leave me so long as I have left you, for those who came before you were only doomed because of their asking too many questions and differing with their Prophets. If I instruct you to do something, then do as much of it as you can, and if I forbid you to do something, then do not do it.” 
The relevant point in this hadith is the words of the Prophet (blessings and peace of Allah be upon him): “O people, Allah has made Hajj obligatory for you, so do Hajj.” Similar reports were narrated by Imam Ahmad and an-Nasai. This hadith is quoted as evidence to indicate that a command with nothing to suggest that it should be repeated means that it does not have to be repeated, according to the books of Usul al-Fiqh.” (Adwa al-Bayan, 5/74)
But if there is something to indicate that it should be repeated, then it must be repeated, based on this corroborating evidence. An example of that is if a command to do something is connected to a condition or cause that makes it required to do it. In that case, what is appropriate to the wisdom of the Lawgiver is to repeat the enjoined action every time the reason for which it was prescribed recurs.
Ibn al-Lahham (may Allah have mercy on him) said:
“With regard to the Lawgiver, He is most Wise and it is not possible for there to be any contradiction in His commands. Therefore if He prescribes a ruling and gives the reason for it, we know that He has prescribed that ruling to be done every time that reason recurs. And Allah knows best.” (Al-Qawa‘id wa’l-Fawaid al-Usuliyyah, p. 240)
The hadith quoted above mentions admiration as the reason for praying for barakah; this indicates that the supplication should be repeated every time one sees something that he admires.
Is it a sin if you don’t say it?
As for not praying for barakah, what appears to be the case is that there must be one of two scenarios with regard to the seer (looker):

The first scenario: the seer admires it greatly, to the extent that he fears that he could harm his brother with the evil eye. In this case it is obligatory for him to pray for barakah, because the Muslim is required to ward off evil and harm from his brothers.

Ibn al-Qayyim (may Allah have mercy on him) said:
“If the one who is looking fears that he could cause harm by his evil eye to the object that he is looking at, then he may ward off its evil by saying “Allahumma barik ‘alayhi (O Allah, bless it for him),” as the Prophet (blessings and peace of Allah be upon him) said to ‘Amir ibn Rabi‘ah, when he looked at Sahl ibn Hunayf: “Did you not pray for barakah for him?” – In other words did you not say  “Allahumma barik ‘alayhi (O Allah, bless it for him)”?” (Zad al-Ma‘ad, 4/156)
Ibn ‘Abd al-Barr (may Allah be pleased with him) stated that this is obligatory, when he said:
“The words of the Messenger of Allah (blessings and peace of Allah be upon him), “Did you not pray for barakah for him?”, indicate that the evil eye does not cause any harm if the one who looks at a thing prays for barakah for it; it can only cause harm if he does not offer supplication. So what is required of everyone who admires a thing is to pray for barakah for it. If he prays for barakah, that will inevitably ward off what is feared of harm. And Allah knows best.” (At-Tamhid, 6/240-241)
Al-Qurtubi (may Allah have mercy on him) followed him in that in at-Tafsir (11/401), and it was quoted from him by Ibn al-Mulaqqin in at-Tawdih (27/401).

The second scenario: If he is not known to have an evil eye and he does not fear that he could cause harm, and he is not worried about harming his brother with his evil eye , then in this case it is still prescribed to pray for barakah, because that is part of showing kindness to his brothers, although we have not come across anyone who says that it is obligatory in such a situation.

For more, please see these answers: 299965 , 7747 and 33663 .
And Allah knows best.