Estimating the time a particular task will take is difficult. I tend to overestimate in the short term, thinking, say that a half-hour task will take me an hour and a half. On the other hand, I think I can finish a book by the end of 2010 while maybe more realistically it will be the middle of 2011.
There is a danger in overestimating, which is that we might procrastinate more if we don't think we have time now to address a particular task. If we had a more realistic idea we might just do it now. People also procrastinate by underestimating: "That will only take me 15 minutes so I can wait to the last minute." In other words, both the sense of dread at the enormousness of the task or an underestimation can lead to the same result: procrastination. Either way, it is important to make sure that, even if we don't estimate correctly down to the minute, it doesn't get us into trouble. For example, we might devote 15 minutes to simply beginning the task and seeing how easy or fast it's going to go.