How long in distance and hours is the drive between Mt. St. Michele and Versailles? What are the best routes? Not necessarily the fastest.
Check on google maps. You can choose different routes, on national roads or smaller country ones.
you may also want to pad that time too.
And remember that for Le Mont Saint Michel you can only drive as far as the parking some distance away on the mainland. From there you have to get a navette to the foot of the mount which will add at least half-an-hour to your journey time in each direction, possibly more.