When making project estimates, divide by two, then increase to the next higher unit.
For example:
If you're told it'll take 5 minutes to fix, it'll actually take 2.5 hours.
If you're told it's a 4 day job, it'll take 2 weeks.
That two week overhaul? That'll be one month.
That one month redesign? Actually half a year.
That two year development project? One decade.