If this is your first time, you really should allocate as much time as possible. It's a helluva lot easier to decide at 1AM that you've had enough than it is to realize at 2AM that you haven't had enough.
(do I get the award for worst sentence structure of the week?)
I was planning on getting the express, but I've read in other threads here that opening night lines are not as bad. Any advice from you grizzled veterans?
We went to the Busch Gardens event last year (a Saturday night, but we had no choice) and I was miserable with the lines.
Best of luck! Let us know how it goes!