HereThere is a blog that I was sent towards of estimation:
http://www.joelonsoftware.com/items/2007/10/26.htmlblog entry
This blog that outlines how to keep a record of how accurate your previous estimations have gone, and then next time you say to someone "it'll be 2two weeks", you can look at your previous history and see how long it actually took last time you said "it'll be 2two weeks".
I haven't tried it myself, but I'd like to, to see how accurate my estimations are.