Hofstadter's Law: It always takes longer than you expect, even when you take into account Hofstadter's Law.

You need to calibrate your "optimism factor" as most of the time you'll estimate by the wrong amount. I chalk this up to people being optimists, that is, ignoring a lot of the junk involved in completing a task that they'd rather not do: Meetings, documentation, integration issues, testing and so on.

For most people it's around 2-5 but for some as high as 10 to 20.

You might feel like an idiot when you tell people "Hm, change the color of the logo to a different shade of green? Six hours." This is probably how long it will take, though, for that so-called "five minute fix".