Several have commented on that ( how long ) and the answer is it varies. I remember one of the 101st ( don't remember who , but one of the now famous front line ones ) who carried an M1Garand. He jumped with 80 in his belt and a couple of bandoliers ( 2x6x8 = 96 ) around him , so about 176 rounds , plus maybe a few clips scattered in his pockets . He said this would last him for maybe 3-4 days before resupply. He said that sometimes he ran low , but he never ran out.
Several books I have read about big mass charges of the enemy show that these would usually last a few minutes and the writer stating that afterward they'd have 8-10 empty clips around them. So it would vary , but these men do not seem to use as much as the newer troops do. As a matter of fact , there was an article about the older men participating in training exercises with the younger troops. They would usually win because they were the defending force , they were underestimated , and the other side ran out of ammo.
Chris