When will be your project finished? How long does it take to complete it? Any idea how do you estimate such timing on the spot? [duplicate]

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Possible Duplicate:
How to respond when you are asked for an estimate?

In my company, i have a strange managers, they always asks me before starting how is it going? And how long it will take to finish? From Day 1 to end of the project life. Behind the screen they do not have any idea what language i will need to use. I was myself forced to use PHP, C, Java, Vala, D, Bash, many other platforms etc etc, where i was only a Visual basic 6.0 programmer.

But it takes me lot of time to complete a project, because i care about it before releasing, requires lot of testing etc etc.

The problem is, i still do not have any best answer when someone asks me "how long do you think it will take to finish this project?", although i have done lot of projects. But i can not answer such questions, because in my mind its never finished, i always wanted to do more and more optimized code even after release.

I would like to know how you guys tangle this question? (as pro-programmer way)