I almost always offer both sides at each feed, usually he will feed on one side for say, up to 20 minutes or so, then take some from the 2nd side but not drain. You do always have to start with that side the next time around. I don't really time the first, I let him decide when he's done, usually he will stop sucking so I just pop him off (he's not one to pop off on his own), I can also tell when that first side feels more empty when he's near done. With my dd I double side fed too and she bf until 18 months.