First of all, most people on this baord will never shoot 20,000 rounds through their all their weapons put together, so most companies through out a BS number that really means nothing.
Secondly, the way you fire and treat the weapon will have more to do with how long the barrel lasts than the amount of rounds you put through it. If you do all your shooting at a bench shooting, slowfire only range, the barrel will last much much longer. If you are running CQB drills, don't expect it to do as well as the guy who only slow fires. Either way, it isn't a big deal to change the barrel.
I have a Bushmaster barrel that I beat on without mercy (loads of CQB drills). It still shoots well enough to make head shots at 100 yards without any problem, and while that isn't good enough for many people, it is more than good enough for me.
I guess my answer is to not worry about it.