On my bushmaster w/ 14.5-in 1:7 chrome-lined barrel, I have yet to cause any damage that I'm aware of. The most torturous heat-up I've put it through would be 3x 30 round mags one after the other. Within each mag the average shot timing was probably around 1 second a peice (some faster, some slower), and probably about 20-40 seconds between mags to swap em out, wipe the sweat off my brow, re-acquire the target, and start up again. I can't imagine I'll ever feel like putting 90 rounds through it any faster than that on a semi-auto gun (of course, I say that now, but I bet once I learn to bumpfire I'll be abusing it worse).
I was under shade, and the barrel did not appear to be glowing, and the handguards weren't getting soft or smoking either. It was beginning to feel uncomfortably hot to hold the handguards sometime during the 3rd mag, but not so much that I actually took my hands off of them. I let it sit and cool off in the open air for about 15-20 minutes before I bagged it up, for fear of burning/melting the rifle bag with the hot barrel. Next trip out a week later it was still shooting the same groups it did before.