About an hour I think.
I camped in the centre once and walked about 10/15mins to get to either end and 5/8mins to cross it.
I may be way wrong as this was 20 years ago. But if you don’t get another answer this will have to do.
I really enjoyed my time there, but I didn’t get a chance to snorkel the reef (which many talk about as HK’s best). So I’ve always wanted to go back.
There’s an island hopping ferry which leaves from Chinese University Ferry Pier every morning (0900hrs I think), and returns from Tung Ping Chau at around 1700hrs.
If your familiar with the ferry and facilities on the island, it’s probably a pleasant and easy trip. But it’s a bit daunting if like me you’ve never done it. I was taken in a landing craft and stayed in army barracks.