Why is it that whenever I have something I want to do - something that I'm really excited about - it always seems to take far longer than I imagined? Then there are those other tasks - the ones that I really dread doing - which never seem to take quite as long as I imagined.
This human trait of underestimating the things we love to do is perhaps the leading cause of overcommitment - thinking we can do everything, and then ultimately achieving little of what we set out to do. Conversely, the trait of overestimating is perhaps what holds us back from attempting those things that are not as difficult as we might presume.
When you can't trust your own estimations, what can you trust? I've recently started to learn that it all comes down to comparables. If you've done this before, how long did it take you last time? If you've never done it, how long did it take someone else? How similar were they to you? What characteristics of the environment are different to other those comparable scenarios? What is the estimation of a neutral third party?
With a few comparables, you at least start to head down a more sane path then what might be in your head alone.