There are so many variables involved, it's hard to say how long any barrel will last, regardless of the gun it's attached to. Reportedly, overheating causes premature wear. The Colt manual states that 12-15 rounds per minute can be maintained without serious overheating. One would assume this advice would apply to other ARs as well.

However, years ago, a well-known barrelmaker, Ed Shilen stated that it would be difficult (he didn't say impossible) to damage a barrel from heat alone unless a machine gun was being fired.

I would guess many shooters don't really know how many rounds they have fired through a barrel and estimates may not even be close to the real number, large or small.

Cleaning? Copper buildup, unless a bore is badly neglected, doesn't affect accuracy as much as many think it does. Same with carbon fouling, but once it's built up to a point where accuracy deteriorates, carbon is extremely difficult to remove; far more trouble than copper fouling.