After you know how you work, and you're capable of, deadlines can be set realistically. Still, estimating any amount of work to a specific number of man hours is always tricky.
I always set
unrealistic short-term personal deadlines. It works really well for me because I thrive on challenge. Like today, I woke up thinking: "I will redesign and finalize our new website today." My medium-term deadline being the site going live by Friday which involves modding the WP code, slicing up the concept and etc. Is this actually going to happen? Probably not, but it might. Just like I might beat Kid Icarus someday, or finish Sexy Hiking.
What's the worst that can happen? You fail to meet it, and finish a few days behind schedule? Well, actually... you're still ahead of schedule because you gave yourself 1 minute to run that 6 minute mile.
Give it a shot, especially if you consider yourself a competitive person. Beat the shit out of that deadline so if you actually meet it, you will want to scale a mountain and thrust its severed head to the heavens and cry
"There can be only one!"