Re: how much is acceptable chamber erosion
What Jon said.
I've seen precision 5.56 barrels shot out around 1200 rounds, while some have gone nearly 5,000.
What is your rifle for? If you're trying to put shots into small groups or targets far away you'll notice when your accuracy dies -- little things like wandering groups, unexplained groups with multiple centers, uncalled fliers, and needing increased elevation to make up for lower impacts.
Accuracy decline in chro-moly barrels is usually a long, slow process. Stainless barrels tend to die quickly. Bore-scoping is only a guide.