I can't really say about what's acceptable with 5.56/.223. I'd think that there would be a throat erosion gauge that would measure that readily and that there would be acceptable and unacceptable measurements. I suppose you could a barrel hot enough to warp as well.
Full auto fire would eat a throat up pretty quick. If you shot long enough and fast enough to discolor the barrel on the outside......think what the inside must look like!
I know that 7.62/308 for match standards is around 4-5000 rounds or so. That's for a paper puncher shot in 15-30 round single fire strings. Kinda depends on when accuracy deterioates.