yeah I totally agree with Kristie. The more coverage you have the better you will be at preserving the day, but 6-7 hours is (to me) ample time. You'll have time for your getting ready shots (EXTREMELY important in my opinion), ceremony coverage, formals (or informals as I like to call em), and a few hours at the reception. By this time, there will be several people with a drink in one hand and a point-and-shoot in the other... lots of coverage at the reception, but its always good to know there is one sober one in the crowd taking the spontaneous photos, rather than the 'posed-fun' photos...
my 2 cents (but they are canadian cents so they are pretty strong at the moment lol)