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Firstly, the term for that is "Eye Relief". Next, eye relief is dependent on the optic you're mounting. typically 3-4" will be in the ballpark, but check it to the way you hold your rifle.
Try to mount it as far forward as you can while still being able to see the entire field of view comfortably. If your gun has any recoil to speak of, the forward position will help you to avoid getting whacked by the scope.