Everything will always take a lot longer than you think. For example, the last two days we spent shooting a video. I honestly though the thing would take a few hours, but the director of the video told me, you should block off two days. Boy was he right about that.
I put in a rental for two days for some lights and when I picked them up, I asked the guy, if we finish early today, can I bring them back and not be charged for the second day? He said of course, but on the second day, I was barely able to return the lights before they closed because we ended up shooting til the deadline. Something that appears so simple on the surface, generally ends up being a lot more complicated the deeper into it you get.