Think of it this way : If the contractor says three days, and it takes
a week, he may have to deal with an angry customer, whose home is his
work site, for four days. If he says it'll take a two weeks, and he is
done in one, then usually the customer is happy instead of angry. It
took the same amount of time, but one customer was very happy, and the
other was very angry. I'd always overestimate a bit. Something
unexpected always happens, and you should have at least a day of fudge
factor added in, more for big jobs.

Dean G.