Let baby dictate feedings. Feed for at least fifteen minutes per side or until baby decides to be done. Your milk supply will come in better if you let baby decide when he's done. If he pulls off wanting to burp, burp him and then try him in the same side. If he refuses it but still acts hungry, offer the other breast. The longer he feeds on one side, the more hind milk he gets, which is super fatty and good for him. It also tells your boobs to make more milk so supply isn't as much of an issue.
With my son (18 months) I have always nursed until he decided he was done. And if I fed him on one side for one feeding, I would do the other side on the next. Just make sure you have a breast pad on the side you aren't nursing on because when one side lets down, the other side does as well.