I like to use Bob Vila's rule: Take your estimate and double the units of measure. So if you think something will take one minute to complete, your estimate should be two hours. If you think that a project will take three hours, your estimate should be six days, and so on. Now another good rule of thumb is to use Scotty's (from Star Trek) method for estimating; multiply everything by a factor of four. Something that should take two weeks to complete will actually take eight weeks.