There is no responsible fixed timescale for every re-roof. A useful programme explains the stages of your project and what could change them, rather than giving a quick promise before the scope is understood.
Separate site work from the whole programme
Ask when access equipment will arrive, when stripping is expected to start and when removal and clearance are planned. The dates scaffolding is present may not be identical to the days the roofers are working. If you have access needs or a deadline, discuss them before booking rather than assuming the programme can move around them.
What can affect the duration
Roof size and shape, access, material availability and the amount of chimney, valley or roofline work all matter. Weather can affect when exposed work can be carried out safely. Concealed timber defects may also change the scope once the covering is removed. Ask how you will be told about changes and how any additional work will be agreed.
Ask about protection between stages
You should understand how the contractor intends to manage the roof while the work is under way and who to contact if there is a concern. The answer needs to suit the project and the weather. A generic promise that nothing can ever interrupt the programme is not a substitute for a considered plan.
Finishing properly is part of the job
Our approach is to allow the time needed for the agreed work. We would rather spend longer on a detail than rush it to get to the next booking. The finishing stage should include checking the completed work against the scope and discussing any outstanding items, rather than treating the last visible tile as the end of the conversation.