I like to steep a tea until it is no longer interesting to me.
For most oolongs, that's usually 6-10 steepings depending on quality, sometimes over a period of days. I usually steep 4 times and set it aside for later.
For senchas, it's usually 3 steeps, but sometimes with a really good tea I'll go as much as 5 (like the YM), so I generally don't start those until I'm sure I can get the most out of them for my money. Most of the time I won't start a tea like that unless I have the day off and can stretch the steeps out a little so I don't feel like I'm just forcing myself to drink a bunch of tea before work.
Of course, with the hot weather later steeps can be used for iced tea in a pinch.
I'm with Chip though, I never start the same tea twice in a day.
Started the day today with a safe dose of the maojian green. About to head home and finish of this DC come hell or high water. Hopefully the leaves are still good... that will be an adventure in itself. More organic 2010 kabusecha on the 'morrow before another fun-filled day of work
. Monday I have off so I can start my day with Yutaka Midori