I agree with parent post. I'm asked this all the time because I'm a lead developer at the company I work for. If I can't provide an estimate on the spot, I tell them I will have to get back to them about an estimate. I try to buffer for time for bugs and QA. Bugs are things you just can't account for. You may have none, you might haave tons. You have that one bug that takes forever to diagnose, or you might have 20 bugs that you can knock out each in minutes.

Just try to be fair in your estimate; fair for you and fair for the business unit. If your estimate is something that the business side can't understand why it may take so long, tell them in detail what goes into it. The Business side is the one who wants the product and the estimate, so you have to communicate to them clearly why the time is what it is.