Istikharah Signs
Question

I need your advise on the following, please. What happens if two people who are planning to get married both read their Istikharah Namaz and only the woman gets the message and not the man. This sister saw herself and her future husband as being happy and together and she feels that ALLAH is telling her that they are right for each other. But what if the man doesn't get any sign/feeling/dream whatsoever. What do they do then? And for how many days does one have to read this Namaz. Some people say for 3 days others say for 7.
Praise be to Allah.Authentic evidence for the Istikharah prayer
The evidence for the Al-Istikharah prayer and the Du`a to be recited is the report narrated by Al-Bukhari (1109) and others from Jabir ibn `Abdullah (may Allah be pleased with them both). 
True signs of guidance after performing Istikharah
With regard to what some people say, then he should go ahead with whatever he feels happy with, a Hadith to this effect was narrated from the Prophet (peace and blessings of Allah be upon him) narrated by Ibn As-Sunni according to which he said, If you are concerned about a matter, then offer Istikharah seven times, praying to your Lord, then see what you feel most drawn towards, and the good will be in that.
An-Nawawi (may Allah have mercy on him) said: Its chain of narration is odd, and it includes narrators whom I do not know. (End quote from Al-Adhkar, p. 132)
Al-Hafidh ibn Hajar (may Allah have mercy on him) said: If this were proven, it could be taken as evidence, but its chain of narration is very inauthentic. (End quote from Al-Fat-h, 11/223)
Al-Hafidh Al-`Iraqi (may Allah have mercy on him) said: It contains a narrator who is known to be very inauthentic, namely Ibrahim ibn Al-Bara’.. Therefore, the Hadith is very inauthentic. (End quote from Al-Futuhat Ar-Rabbaniyyah, 3/357)
Understanding the significance of ease and obstacles
The correct view is that when Allah makes something easy for you after having decreed it and accepted your Du`a this is a sign that it is good to go ahead and do it. The existence of obstacles and difficulties is an indication that Allah is pushing His slave away from doing it. This meaning will be very clear when one ponders the meaning of the Hadith of Jabir concerning Al-Istikharah , where the Prophet (peace and blessings of Allah be upon him) said:
O Allah, if You know this affair [and here he should mention it] is good for me in relation to my religion, my life, and end, then decree and facilitate it for me, and bless me with it, and if You know this affair to be ill for me towards my religion, my life, and end, then remove it from me and remove me from it, and decree for me what is good wherever it be and make me satisfied with such.
Ibn `Allan (may Allah have mercy on him) after transmitting the scholarly view that the Hadith of Anas is inauthentic said: Therefore it was said: that after (doing Istikharah ) he should do what he wants (i.e., even if he does not feel that sense of being happy), because whatever happens after doing Istikharah is good
Al-Hafidh ibn Hajar (may Allah have mercy on him) said: Al-Hafidh Zayn Ad-Din Al-`Iraqi said (concerning doing something after praying Istikharah): Whatever he does, there will be good in it. This is supported by what is said at the end of the Hadith of Ibn Mas`ud according to some of its chains of narration: Then let him make his decision. I (Ibn Hajar) say: I have already discussed above and explained that its narrators (i.e., the narrators who added the phrase Then let him make his decision) are inauthentic. But this is still stronger than the narrator of that Hadith (i.e., the Hadith which says, then see what you feel most drawn towards). (End quote from Al-Futuhat Ar-Rabbaniyyah, 3/355-357)
The relation between Istikharah and dreams
Another of the myths that are widespread among people is that after praying Istikharah you should sleep, and whatever good you see in your dream that makes you happy means that what you want to do is good and will be made easy for you, otherwise it is not good. (This is what the questioner meant by saying gets the message). There is no authentic evidence for this as far as we know.
The above does not mean that if you feel happy it is not a sign, but this should not be regarded as the only sign about whether or not the matter is good. People often make Istikharah about something they like or want to do, so they feel happy about it from the outset.
Shaykh Al-Islam [Ibn Taymiyah] (may Allah have mercy on him) said concerning the matter of feeling happy about something: If he makes Istikharah, then whatever Allah makes him feel happy about and makes it easy for him, this is what Allah has chosen for him. (End quote from Majmu` Al-Fatawa, 10/539)
Repeating Istikharah: Is there a limit?
There is a difference between making feeling happy the only sign and making it one of several signs. There is no set time period for Salat Al-Istikharah; it is permissible to repeat it more than once, but there is no limit to how many times. The person may offer the Du`a before or after saying the Salam (when concluding the prayer). 
And Allah knows best.