I program for 20 years now and I think that what many people do wrong about these estimates is that they give them too early. The truth is that for many project

I program for 20 years now and I think that what many people do wrong about these estimates is that they give them too early. The truth is that for many project the only truthful answer you could give someone on the question hoe long it takes is: "That depends on many things some of which I don't know, some of which we both don't know and some of which potentially nobody knows." After that you should say: "In my experience it takes betwern x and y weeks, with a lot also depending on how responsive your side is." Time estimates are always hard, not only in programming. And outside of programming one of the main insecurities is customers changing the plan or wanting adjustments. This is the side you can't really control, so it is best to get a feeling for the customer, their communication patterns and their expectations early on and factor it in. The other insecurity is tough problems you encounter during the programming phase. How well you can deal with those depends a lot on how experienced your programmers are and how much they were involved in the inital process. The truth is that the latter insecurities make up a main part of the whole thing and it has to be okay to tell a customer you can't give them an estimate before you know some more details.