If it takes you longer than you said it would, you're shit at your job, but if you get it done in a fraction of the time you claimed, you're a hero and obviously they don't pay you enough.

As a former retail computer repair guy, one of the quotable bits of wisdom I have in common with an engineer friend is this: "Whenever you get or give an estimate of money or time, double it. Then double it again. If you don't, someone is going to be disappointed."