A straight roller swap on one side of a double door, once we're on site, usually runs under an hour - it's removing panels and brackets, not rebuilding the opener or retensioning springs. Doing all ten rollers on a double door, nylon or steel, takes longer only because there are more brackets to work through, not because any one of them is complicated.
The variable in Bethlehem isn't the repair, it's what's underneath it. On a house built around that 1960 median, we sometimes find the track itself has shifted slightly out of plumb over sixty years of settling, which means a few extra minutes squaring it before the new rollers go in smoothly. We'd rather spend that time on site than send you off with rollers that grind again in six months because the track was never true.