Ev, in my opinion I would say a day. That's how long it took me to do my abstract for my upcoming major conference. I had a fair idea of what I wanted to say, but I had to refine it, refine it and refine it. Maybe I'm a bit anally retentive but I thought about every word, the structure of it, and then formulated a catchy title to make it stand out from the others.
I am perhaps a little slow though, when it comes to these things. I wouldn't have thought more than a day though.