I find that reusing a black tea bag that's sat for a while (say an hour) just doesn't taste right. It gets a really strange taste (for me). So I only use it once, and that's it.

The only exception is when my boyfriend wants a cup. He likes his tea super weak, so I just dunk the bag a couple of times and then I'll steep mine longer.

I have a Scottish co-worker who'll use the same Tetley bag all day, which I find a tad too much.

The way my parents drink green tea, they do it the way they do in Japan. They'll put quite a bit of the leaves in a small pot (and I mean small, probably holds only about 12oz). They'll steep for a short time, like 1 minute or less, and pour it out. The cups they use are small, no more than 2oz, I think. But they keep adding hot water to it and just keep pouring it out.