I agree, things always take way more time than I expect and the pomodoro technique has really helped me in this area more than GTD. Which I started before I had even heard of GTD.
http://kouroshdini.com/2009/11/23/gt...-the-pomodoro/
I on the other hand really like putting times on everything even if some big projects are next to impossible to put a very accurate number on. It helps me a little in not being a perfectionist on things that don't need it. The biggest help for me is for daily "sharpening the saw" (Steven Covey) actions that are timed events to make sure I am increasing my skills in various areas daily by a set amount of time.
I put those all in a context and use a perspective that sorts them in numbered priority and then I highlight what is left to see how much time is needed. All are listed in 25 minute chunks for me, so 50 minutes would be an hour two five minute breaks. That is what seems to work best for me.
Reading this post and replying would be part of those "Sharpening the Saw" goals that are timed.
Interesting to me to read how people use the time feature.