Personally, I am a master at underestimating how long it will take me to get things done. "Yeah, I can do that in two weeks," almost always turns into... more than that.
Another important thing to consider is to not underestimate the amount of progress you're making, either. Sure, the NPC may have needed work, but it's still forward progress which, in itself, is awesome.
Thankfully I’ve been rather lucky to be over estimate the amount of days things will take for the most part as far as deadlines are concerned. Then again I’ll be grinding crazy hours till sunrise sometimes to make them happen.
While it is forward progress. It seems like it’s never ending the things that need to be done.