Because I was damaged when I fed my son, I only fed on one side to give my other side a break. At about 4 months, his weight gain slowed, and I ended up feeding on both to make up for it, and then had to take supplements too. With my daughter (3.5 months), I feed on one side, mainly out of habit, and her weight doesn't seem to be slowing at all right now. I actually have about 200 ounces of EXTRA milk in the freezer, even though I was told that my supply would slow down if I only fed on one side. It seems that your body just realizes what you are doing and accommodates. For the last meal of the night though, I do feed on both sides so that she gets a bit more to last through the night since I usually have less milk at that time.
As to when to switch, it is just a cue thing. During growth spurts, I stick with one side until she gets too mad (to stimulate that side), and then offer the other if she is still hungry. Helps both of us that way. Then go back to just one side when she seems satisfied with it again.