For a .308 based round and especially for maneuvering in and around tight areas, I prefer 20" barrels and no more than 22", assuming the contour allows the rifle to balance well.
From a powder efficiency standpoint, no more than 20", though as mentioned earlier noise and blast can be a factor especially in a blind. For burning 40-50 grains of powder, longer barrels don't have any advantage, IMO. Even more so for .378" rounds.
That said, two of my three main hunting rifles have 25.5" and 26" barrels. I need to have them chopped at some point.
