The full trail would take a fit rider, who knows the trail and riding fast, about 2 hours. I've taken very new riders out on it, and it's taken over five hours. If it's hot, bring a minimum of 3L of water. Also bring something to eat.
Bear in mind, I'm talking about doing the whole trail with lots of stops. You can always turn around any time. You can also choose between 3 start points - the Mui Wo and Pui O ends, or catch the short ferry from Mui Wo to the prison which is halfway along. This ferry is very infrequent though- check with Jacky at Friendly Bikes for the timetable.
Riding over the hill from the end (Pui O) back to Mui Wo via the road involves a massive (well, by HK beginner standards) hill climb. Can be done in <20 minutes if you're in good shape, but not recommended for beginners. Instead, you can flag down a taxi which can take a couple of bikes in the trunk. There's a slight surcharge for bikes- about $10.
Anyway, don't let the complicated bits of my post put you off. Lots of new people rock up, hire a bike and just start riding, and have a good time of it. The 3pm finish isn't terribly practical though, specially with such a late start. Better to make a whole day of it, and finish with a meal at the China Bear afterwards.