There is no length, or at least when I was invited to experience there was no time limit (no wait to enforce one though) unlike the SeaVenture which is opening later this summer it is an open reef with "shelf's" to stand on and deeper areas to snokle through.
during random times of the day they will have trainers out to have guests feed the rays some shrimp as well!
I saw Rays, sharks (behind glass) and some tropical fish in the large environment.
It was a fun experience and other than knowing how artificial it is and the life guard positions it could easily pass as a tropical beach, I had tons of fun (though I was invited and didn't pay for my brief visit)