A straight swap on a single door, old unit out and new one in, generally runs a few hours once we're on site. Where it runs longer is when the opening itself needs rework - and in Unicoi, with most homes built around 1981, that's common enough that we plan for it rather than treat it as a surprise.
Framing from that period sometimes used lighter jamb material than what current doors are engineered to anchor into, especially on double-wide openings. When that's the case, we reinforce the jamb before the new door ever goes up, which adds time but isn't something we'll skip to hit a faster number. A door hung on a weak frame comes back as a service call within a year, and that costs the homeowner twice.
Weather plays a role too. In a cold-moderate climate, winter installs mean we're watching for ice on the slab and cold-stiffened seals, both of which can slow a job by an hour or so if conditions are bad that day. We won't rush a weatherstrip seal in freezing temps just to finish early - a bad seal in a house already working against 40-plus-year-old insulation isn't worth the time saved.