A straightforward swap - same size opening, header already sound, wiring already correct - runs a normal single-day job. Where it slows down is exactly what we flagged above: if the header needs doubling up or the framing needs to be brought back to square before the new door can be trusted on it, that's additional time, and sometimes additional material we don't have on the truck that day.
Be honest with yourself here too. If the opening is badly out of square or the framing's gone soft, sometimes the right call is a second visit to rebuild that section properly rather than forcing a new door into a bad frame on day one. We'd rather tell you that up front than have the door come back to us in two years with the same problem. On houses built this long ago, that's not a rare outcome - it's common enough that we plan for it.