lisa,
on average, the afternoon parades are about 20 minutes in length, give or take a few minutes. As for how early you should stake out a viewing area depends on how good a spot you want and where you want to view from. I have seen people staking out spots on main street two hours before the parade. we have always used the 15-20 minute rule and found that we can get just as good a view, but, maybe in a little out the way place instead of the more mainstream areas. It really just depends on how bad you want that "picture perfect" spot.