Eventaully the throat wears. Cracks start to propagate and erosion increases the diameter of the lands. To set back a savage barrel you need to have enough of the thread diameter on the OD to have room for the barrel nut(unless you are using a barrel nut delete type tube). A bore scope or chamber cast might tell you how much needs to be removed to minimize the amount of setback is required to get to fresh rifling. Anything else would be random weather appropriate or not, while I am certain some smiths have an amount the smith usually goes by. Asking yours would be a good place to start. On longer barrels some even remove about 3/4"-1" off of the muzzle end as well .