[youeseh]

TL;DR: Lock down the scope before providing estimates. Always double your estimate because developers are damagingly optimistic about their ability to get things done.

Slightly longer version: If the scope isn't locked down at a granular enough level, one where each unit of work would take no more than one day, then you can not trust the estimate.

If you as a developer provide or agree to an estimate before the scope is locked down, then you're doing this at your own peril. That estimate will be used against you at every opportunity. It will be used to pressure you into working for free (overtime isn't usually compensated for in full-time positions). Your personal relationships or chances of getting into one will suffer. You'll begin to hate the people you work with. You'll get fat and / or ugly. Remember Milton? Never forget Milton!

Being asked to provide an estimate prior to scope is more of a negotiation tactic than a serious attempt at accomplishing valuable work. If you thought you were making $x for y hours of work per week, well guess what?! Because of your premature estimate, you're now making $dx where d is less than one and hopefully above zero.

Okay, so lets say that you were able to lock get the scope locked down. What now? Now you can provide an estimate by adding up the estimates for all the granules of work you'll be doing and then you'll want to double that number. Why double? Because most developers are incredibly optimistic about their ability to deliver quality code. You're probably not adequately including time for QA and changes anyway. The last 20% of any project often takes 80% of the time, so double those estimates.