It will probably take you about 1 hour to drive from Playa Del Carmen to Xel-Ha by bus. ( Remember you have to wait for everyone to have everything and everyone and get on the bus!!! Can be annoying at times!!!) I am guessing you will have about 3 hours at Xel-Ha.
This place is awesome. We were unsure as to what to expect last year but were very impressed. It is very organised and clean. If you have your own towel and snorkel gear you might want to take it It will definitely save time in the line ups for the rental equipment.
Enjoy, I know you will.