Everyone in Vietnam, well the soldiers in the field at least, were given a tag that only time would erase. There was not really a specific amount of time you had to serve to shed the title of FNG it just kinda went away with experience. Life in the bush was always a little more dangerous when new soldiers rotated into the company. The new guys were really nervous, didn't know what was going on and needed some guidance. Once they really became part of the squad they could be trusted and they lost the title of FNG.

Oh yea, FNG stands for 'Fuckin' New Guy'.