Green teas I just brew out until they're done (or within a few hours of the initial steep). The amount of steeps depends on the quality of the tea.
With Oolongs I steep 4 times then usually put the vessel aside for later evening steeps. After the first four good cups, again, the amount of steeps depends on how well I timed the subsequent brews and the quality of the tea. Cheaper teas become boring and I don't like forcing myself to drink tea flavored water. Higher quality oolongs will hold my interest for longer. Sometimes, if I have something I really like, I will flip the whole clump of leaves over in a gaiwan and repack them to get more flavor out.
Then again, some days I just brew until the leaves are exhausted (or to the point where it's not worth it anymore to me). Those are usually the days I break out the high end stuff and want the tea to be as fresh as possible. On these days I brew in a small vessel (I've found 80ml to be about ideal- more than just a sip per cup, but small enough to where I won't be so full of tea after the session I feel sick), and I will stretch my tea experience out to 2 or 3 hours or more! I really love days when I can do that and just relax and drink tea. Very cathartic...
In general though, my rule of thumb is the leaves are good for about 12 hours after the last steep, and 24 hours total after the first steep. I'm flexible with that, though. I'm amazed at how long people will brew out the same leaves. I've heard of weeks of brewing the same leaf! I don't know of any tea that could hold me captive that long, and I'd be worried about the risks of leaving wet organic matter out on my counter that long and consuming it
High-fire TGY out of my 80ml gaiwan today. Rainy, cold days always make me crave roasted oolongs.