Steep it until you like it: take however much tea you expect to use for that pot, add your water, wait maybe 20 or 30 seconds, and pour out a little sip into a small cup, taste. Every 15-30 seconds, take another little sample. When you like it, stop. It varies so much by tea and drinker--I like my teas mostly weaker, but for some light roast oolongs I push them a lot more. You can't go wrong with some tasting along the way.
Compared to most green teas, I tend to brew my first infusion of a light oolong at least twice as long, but I don't cut down as much on the infusion time for the second infusion. E.g., for a sencha, I might brew 1 minute, 30 seconds, 1 minute, 90 seconds, 3 minutes, etc; but a series of infusions of oolong might be 1 minute, 1 minute, 90 seconds, 2 minutes, etc. I'm not sure from your question but it sounds like you might be brewing 'western style'--1 infusion from a batch of leaves--and in that case, the green oolong might go 3 or more times as long as the plain green tea, because a good green oolong has far less bitterness to worry about.