Hi S.! I have a 3 month old son and he only breastfeeds for about 20 minutes (10 minutes on each side). I have him on a 3-4 hour schedule. For example, his first feeding is at about 630 a.m. then I keep him up until about 8 a.m. and then he goes down for a nap and then wakes up again at about 930 a.m. and the process repeats itself. This has been really helpful so he doesn't fall asleep on the breast. You can also tickle his feet or undress him to keep him awake. I talk to my son when he starts to dose and tickle him and it typically keeps him awake. He probably gets annoyed with me (and at such a young age but what are Moms for?)!
You may want to check out the book BabyWise (you can get it at Wal-Mart), it teaches you how to get your baby on a feeding, wake, and sleep schedule and it really does work! I have used it with both of my sons and my 3 month old has been sleeping through the night since he was 4 weeks.
But as the other Moms have said as long as your son has enough soiled and poopy diapers per day then you can gage that he is getting enough to eat.
Good luck!
T.