I normally don't recommend under 6 hours of coverage. With my experience, time on the wedding day goes SO much faster than you'd expect, and things can begin to feel rushed if there's less than that. The only time I would go with less time is if there's no formal reception.
Also, as a quick side note: If you have an awesome photographer, some of the best images are captured later in reception when everyone is on the dance floor having a blast! It's not even comparable to guest images. So if you want to remember what an amazing time you and your guests had, I would definitely recommend having your photographer there to capture it all.
8-10 hours is much better for capturing all the special moments of the day. From the getting ready and detail shots, to the ceremony, formals and fun wedding party shots, romantics, and reception... You're putting a LOT into planning and making your once-in-a-life-time celebration come together. I can pretty much guarantee a couple years from now you wont be thinking about the little extra put towards capturing it properly. But you will have beautiful images to show off and look back on!
I might sound a little biased being a photographer myself, but it makes me so sad when couples come to me after the wedding (either to photograph their trash the dress or boudoir, or as friends) and mention that as their one regret. Obviously only you can determine what's right for you, but if you're on the edge it's usually better to add the extra time and have no regrets.
Wow, that's wordy, lol. Really hope it helps someone out!