The guy who does the work, if he has done anything similar before, will have a good idea of how long it should take.
... but he will try to please you, by giving you an estimate based on having the right tools and the right parts in the right place at the right time, and not finding any ugly surprises.
Things never go that well for me.
Hence, double the estimate.
The result of doing so is very rarely conservative.
Mike Halloran
Pembroke Pines, FL, USA