You could probably use either. Most people won't notice, especially as there is no pronunciation difference.
In general, you can try switching round to the "of" form, to see if plural or singular works better:
A survey in which many boys revealed the behaviour of their girlfriend towards them after some time of their relationship.
A survey in which many boys revealed the behaviour of their girlfriends towards them after some time of their relationship.
The first could be read to imply that there is just one girl, who is the shared girlfriend of the many boys. But pragmatically I wouldn't read it like that I would assume that each boy has one girlfriend.
The second could be read as each boy having multiple girlfriends, but again that doesn't seem to work, so I would assume there are as many girls as boys.
In other words. I will interpret pragmatically and come the exactly the same meaning for the two sentences.
So you could use either. Both are grammatically correct and when interpreted in context have the same meaning.