My DD wanted to feed for hours, and most of the time she was half-asleep. When she was just about a month old she lost weight, and we found out she wasn't feeding effectively. By then my supply had dipped, and it took us a looong lime to sort that out.
Listen for sucking, or feel for it on your baby's neck, it should go suck-suck-swallow, most of the time (not necessary when baby is trying to feed to sleep - provided he/she doesn't try to go to sleep all the time!). But if the baby feeds, say 60 min, and 35 of that is very effective, that is probably fine. However, if the baby feeds 5-10 min, and then slows down, there may be a problem.
What you can do is to try to keep baby's interest in feeding up, tickle under the chin, under the foot etc. When nothing else works, take baby off and switch breasts (if the active feeding was 20 min or more, you may want to change nappy or take clothes off here, as well as try burping). The faster flow on the next breast should get baby started again. Do the same to try to keep baby feeding well, then change breasts again, if active feeding isn't 45 min or more (changing nappies here or try potty). You can swap up to six times or more (we did 4, and fed for about 1 hour). This encourages your baby to feed better, and your breasts to feed more.
Have a look at some of Dr Jack Newman's stuff about effective sucking and switch-nursing:
http://www.drjacknewman.com/help/Protocol%20to%20Increase%20Breastmilk%20Intake.asp
On the other hand, some babies do feed for very long on one breast, possibly 2 hours or more in the early months. It can be normal. But as you suspect ineffective sucking and possible low supply (a common result), I'd check it up. Know that it can be sorted out! We got the supply up, DD is a healthy 3 yo and still breastfed!
Good luck!
Please feel free to PM me any questions, or just to talk. I know how worrying it is.
