Further than you can shoot accurately
400-600 yards give or take.
A 20" bbl will buy you a little more velocity which will in turn give you a little more range, but the tradeoff is needing a stiffer barrel to get the same inherent accuracy. The difference between the 16 and the 20 using the same ammunition is maybe a hundred yards give or take at the same level of accuracy.
Note: That's not the maximum range, that's the accuracy range.
When I was at LRM (Long Range Marksmanship) at Fort Campbell in 2007 we trained to shoot 700 meters with a M4 and 1k with our M14s. The LMR Instructor hit a man sized target at 912 meters with a standard M4 with a ACOG. I ranged the target myself with a laser range finder. It wasn't any super sniper move or anything. We had to spot his rounds and walk him in, but still that's a h3ll of a shot.