It depends on how much you extracted first time
A three-minute western steep pulls out most of what the leaf has in one go, so a second cup is noticeably thinner and a third is barely tea. A fifteen-second gongfu steep extracts a fraction, which is exactly why the same leaf yields eight or ten distinct infusions. The leaf has not changed; the extraction has. Anyone finding that their tea 'does not re-steep' is usually brewing western-style and expecting gongfu results.
Rough expectations
Western brewing: one good cup, sometimes two from a robust whole-leaf tea. Bagged and CTC tea: one, genuinely. Japanese green tea in a kyusu: two or three, with the second markedly shorter. Gongfu with rolled oolong: six to nine. Gongfu with compressed dark tea: ten to twenty, and boiling the spent leaf afterwards still yields something. Whole leaf, tightly rolled leaf and compressed leaf all last longer than broken leaf, because they open more slowly.
How to keep leaf between infusions
Drain it. Leaf sitting in water keeps extracting and the next infusion starts already over-extracted and flat. In a gongfu session, pour off completely every time. Between sessions, leave the leaf in the vessel with the lid off so it can breathe, and come back to it within a few hours — same day is fine, next morning is a gamble. Wet leaf left overnight in a warm kitchen is not something to brew.
When to stop
When it stops being interesting rather than when it stops producing colour. Colour persists long after flavour has gone, so a pale infusion is not necessarily finished and a dark one is not necessarily good. Extend the steep considerably — a minute or two rather than seconds — and if that produces nothing but thin astringency, the leaf is done.