This is something you DO need to address, and sooner rather than later. Many nursing consultants will tell you to let the baby dictate completely, which is not what's in his best interest or yours. I followed that advice, not knowing any better. What the other mom's said about the baby using you as a pacifier is so true, but I was never able to figure out whether he was nursing or playing. As for falling asleep on the breast, yup, he did that too, but I could not keep him awake no matter what tricks I tried, and he'd fall asleep with a tight latch on me...which I misinterpreted as him still wanting to eat. Over the first month and a half, we got into a horrible cycle. He would nurse for too long. I would get dehydrated and exhausted, and consequently not produce enough milk. He would nurse longer because I wasn't making enough milk. I would get more exhausted and dehydrated....At 6 weeks, the pediatrician said he was becoming severely underweight and I had to start supplimenting with formula. Thank G-d, we caught things in time and he's back on a healthy growth curve...but I'm still upset about everything we went though because the current lactation fad advice is so out of balance.
Based on what other moms have told me, it sounds like 20 minutes on each side is a good upper bound, even for a slow nurser. If he's taking longer than that, he's probably playing around - and it's completely legitimate for you to teach him not to do that.
Good luck with this. I hope things improve soon.