Hammer forged is alleged to last much longer than cut or broached rifling. Conventional wisdom is that hammer forged is not as accurate as cut rifling.
When is it wearing out? Generally when your accuracy with a known load gets 2X worse (1" groups become 2" groups) the barrel is in question. There is really 3 basic parts to the equation. The throat, the rifling and the muzzle.
The throat can become eroded from high powered loads. Some chamberings seem to more succeptible to this (.220 Swift). Improper cleaning or not enough cleaning can wear the throat. Corrosive ammo can be a cause.
The rifling can become worn from A LOT of shooting. This really takes many thousands of rounds. The rifling can become fouled with copper. A friend had a tack driving 6mm rem rifle that started to shoot clover leafs instead of single holes and he thought the barrel was shot out. At my behest he plugged the chamber and filled the barrel with Shooters Choice solvent, Plugged the muzzle and left it alone for 2 weeks. At the end of 2 weeks he removed the plugs and the barrel was full of a solid, green 6mm stick of goo. He cleaned, refilled and repeated. After 2 more weeks the green mixture drained out. He cleaned and shot. One hole groups again. Fouling can be so severe it effects accuracy.
The muzzle can become nicked or worn. Careful cleaning from the chamber end with the proper tools can prevent this.