When he is killed off,will another decent buck move into the area quick,or does this take some time.The buck I have been hunting was hit by a car yesterday,he was the only one i saw that I would consider a shooter.

A younger buck will immediatley move up into the dominate spot until a more mature buck find the area to be 'available', which often happens during the rut. The bigger bucks won't always relocate andmake this their new core area, but they will temporarily take advantage of the easy pickings with no big buck around.