In the 70's a Brazilian programmer told me his method was to make his best guess how long something would take, double it, then promise that amount plus-or-minus 50%.
The one I heard was estimate, double it, and move to the next higher unit. 2 days? Nope, that's 4 weeks.
The one I heard was estimate, double it, and move to the next higher unit. 2 days? Nope, that's 4 weeks.