When should I rebarrel a hunting rifle?

[hbp]

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.