Amy:
Epcot's Flower & Garden Festival has been the highlight of two of my Disney trips, including our visit for Spring Break last year! You can take as much time as you want to enjoy the extra gardens and topiaries, but I don't think you would need to allot yourself much more time, unless you were a gardening enthusiast.
Almost everything associated with the festival, from its Butterfly Garden to the concerts at the American Pavilion are included in your Epcot admission! The displays are a feast for the eyes--and nose. Topiaries of Disney characters like Beauty & the Beast dot the landscape as well as familiar favorites like Minnie and Mickey. My favorite display is the array of Japanese bonzai gardens on loan from various collectors. Take the time to look at their nameplates and ages--some are over a hundred years old!
I took a 3-hour guided tour of the festival with one of Disney's horticulturists a few years back and loved knowing the intricate details of the plants. But be aware, the tour is probably not for non-flora buffs. Unlike it's fall counterpart, the Food & Wine Festival, there are very few ticketed events at the Flower & Garden Festival, so you are "free" to enjoy all that is there! I don't have any information on the 2010 festival yet, but last year's information is still up if you'd like to get an overview
here.
Wishing y'all a magical vacation!
Whitney