Most people don't even know what they are actually asking for when they ask 'how long will this take?'. I always try to tease out whether the person asking is looking to find out 'when will this be done by?' or 'how much effort is this?'. Even when you've figured that out, you need to understand what they think they mean by 'done'. code is changed on my machine? I've checked it in? code deployed to QA and tested in multiple browsers and run through the regression suite? code deployed to pre-production for approval by the client? code approved by client, deployed to production and live? Also, do you want me to take into account the time I'll spend waiting for decisions to be made about how this feature has to work, or just tell you how long it will take me to do it once you've got me the answers to these questions?

One thing I've noticed is that organisations can only normally operate at a certain number of 'decisions per day' (and the larger the organisation, the lower this number). For estimating anything where requirements are not yet firm, or will be discovered during development, that rate is usually the constraining factor, not the development effort.