Every roof is different, and weather sits outside everybody's control. What a roofer can tell you is what drives the number.
What lengthens a job
- Roof size and number of separate slopes
- Steep pitch, which slows every single task
- Complex details: valleys, dormers, chimneys, roof windows
- Scaffold complexity and restricted access
- Hidden timber repairs found during the strip
- Weather — particularly wind, which stops work before rain does
A realistic shape
Scaffold goes up first, often a day or two ahead. The strip is usually the fastest and noisiest phase. Re-covering takes the bulk of the time. Details and leadwork come last, and they are the part it is worst to rush.
Ask for a range with the assumptions written down, rather than a single date. A single date usually means someone has not thought about the weather.