It depends which mountain and where in the day it is. Finish on Alpe d'Huez you need to be there much earlier. Col du Mon when they are going over the Tourmalet as well, I think we were there perhaps 3 hours before the first riders hit it. We rode to the top and then rode down to a nice spot and waited. I'm sure more sober people will give more accurate estimates.
It depends which mountain and where in the day it is. Finish on Alpe d'Huez you need to be there much earlier. Col du Mon when they are going over the Tourmalet as well, I think we were there perhaps 3 hours before the first riders hit it. We rode to the top and then rode down to a nice spot and waited. I'm sure more sober people will give more accurate estimates.