Commuting on my bike means I'm a pro at this question. On a daily basis, I "need" about 20 minutes. A really quick (small) bite to eat and I'm off. However, since I'm pretty much a morning person, I often find myself sitting around awake an hour or two before it's time to leave.

For "special" rides (i.e. competitions, group rides, etc) I do like that extra hour or two, just to make sure everything's ready to go.