1 - 2 horas
I was watching mlb.tv one day and all of the sudden the PS3 just turned off. When I tried to start it back up I was met by the dreaded YLOD (which I didn't know anything about at the time).
It went swimmingly! Got the kit, waited for the wife to take the kids to the in-laws, followed the online guide, and now the PS3 is working like a charm.
Took apart and heated the board (about 2 hours), let it cool over night, applied the thermal past and put it back together the next morning (about 1.5 hours, I had never applied thermal paste before).
If you take your time (definitely more than an hour) and keep all of the parts organized, you shouldn't run into any problems.