It can be done, but it will have to be somewhat "commando style". We went last year and got there upon the opening of the park, and stayed until they kicked us out. We felt like there were still some things we wanted to see and do.
If you are mainly interested in the rides, you can certainly hit all of them in one day. But they also have a lot of shows, and a train ride, etc.
I would say that one day in any given vacation time is enough. But we will plan to put Busch Gardens on our vacation itinerary for most of our future Florida vacations.