My favorite method used to be "think of how long it will take, double it, and add 50%". Err, 2.5x what it will really take. But even that isn't good enough anymore, since I'm getting whacked from nine billion directions at once.

My estimation is usually fairly accurate since I've been doing the same thing for over two years now, the one thing I can't handle is outside requests and changes in priority. So I usually estimate things as "will take 2 days of uninterrupted development time", which usually means it will take a week sharing between tasks, or in worst case two weeks. I'll convey both the 2 day and 1-2 week number to project/program management, such that they are aware the task isn't hard, I'm just doing a lot of things at once.

I'd be interested in other systems people have for dealing with estimates as they relate to multi-tasking. My organization usually has 5-6 ongoing projects that affect me at any one time.

Some folks I know have resorted to a priority queue system, and this works fairly well, though it can be hard for some to accept. Essentially you maintain a list of ordered priorities, and then when some one comes in with a request you let them know where it is in your queue. Thus a relatively simple item may take a month to get accomplished.

Currently, I'm on the "cooperative multitasking" model of estimation. For those that play with OS's, you will know this doesn't work very well. You have to let everyone know there is a very good chance their "process" will be delayed indefinitely.