When should I rebarrel a hunting rifle?

[Klem]

I'd take it to the gunsmith that did the initial chambering and have him check the head space and look for throat erosion.
You may just need to have the chamber "freshened",pushed forward and then rehead spaced. If you aren't doing it already, get a Hornady OAl gauge and keep track of changes in your chamber. That should give you some idea of how fast your barrel is wearing.

Can this be done?

I'm thinking this would be more difficult doing in an AR than a bolt gun. Pushing a reamer in a little further while shaving the barrel extension shoulder to keep headspace seems straight forward. But keeping the gas tube hole at 12 O'clock and the barrel extension feed ramps at 6 O'clock.

Surely it's cheaper to just buy a new barrel.