How do you do very quick (and dirty) estimations for coding tasks? [closed]

Question

So you've just been put on the spot by The Boss. You've got 15 minutes to come up with a back of the envelope estimate for the addition of some new feature. Your boss (fortunately) recognizes that you can't provide an accurate estimate in that time so expecting something that is in the right order of magnitude.

The question is how do you go about giving a estimate in the time frame that is accurate to an order of magnitude?

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Note that this is meant to be a quick and dirty estimate, not something that might be expected from questions like this

Answer 1

Place finger in mouth, lick, wave in air and make up a number based on past experience. Then double it.

Really, its just experience that counts. You imagine what the task entails you doing, and you know how long it'll take you to do that. Double it for unanticipated items. This is also why you never ask junior programmers for such estimates.

Answer 2

The best way is to try a quick breakdown of all of the major sub-components, e.g.

1. Update data model script (3 items in 2 tables)
2. Change input screen (3 new inputs)
3. Check Input (3 new inputs)
4. Update Data.
5. Display results etc...
6. Build unit test

Assign a rough guess on each of these and if you can't think of one put down at least 2 hours, because even the simplest item will probably take at least an hour, but the 2x will allow for uncertainty.

At least you will have thought of all the items you will have to do so it will be in the right order of magnitude as was requested.