I'd heard about 20 mins each side, but my ds was a very efficient feeder, and would never feed for more than about 10 mins, and v.rarely took the second boob.
Others I knew would happily go on for 45 mins each side.
I think it depends on how quick your let down is, how good the latch is, and how much effort your baby puts in, to an extent, so it can really vary from person to person.
IIRC, babies usually come off the boob when they're not getting much, but if he / she is still going strong, I'd leave him for 20 mins, then swap over. That way you can be sure he's getting the hind milk.
It's easy to say, but I wouldn't worry too much about the fore / hind milk thing. Babies are very good at taking what they need, and as long as you don't swap over too soon, I'm sure your baby will be just fine.
At 6 weeks, IMO your baby should be able to go more than 1/2 an hour between feeds.
Try and make sure he's taking a decent feed each time, ie not just snacking. If he falls asleep after a few mins, wake him up again and try to encourage to take a bit more.
failing that, is he using you as a dummy??? Some babies just like to suck on something, adn a dummy could give you a break. They're not recommended during the easly days of breast feeding as they may interfere with your supply, but at 6 wks, your supply should be established enough.
Maybe next time he grizzles shortly after a good feed, try him with it.
Remember too, that he can cry for lots of reasons too, don't automatically assume he's hungry. He may be too hot / cold / tired / overstimulated / dirty nappy -you see the point. If you know he's had a good feed, check everything else before whipping out a boob again!!!
I believe this is now lots of babies end up needing to be fed to sleep. They're tired. They grizzle. Mum thinks they're hungry and whips out a boob. They take a few mouthfuls and fall asleep on theboob. Before you know it, they need the boob to fall asleep. And all because Mum thought they were hungry when really they were tired.
Anyway, I digress. Sorry to waffle on.
HTH