as you said it all depends on what type of hack, If i have free time I can produce a hack in a couple of hours, yet if its a simple hack it can take 10minutes or less.
The Admins reading user's pm hack took me about 2 hours to do.
I spent a couple hours on vBGlossary. It obviously depends on the hack. I'd say that touching the hack up, and writing instructions, and simply making it easily usable for everyone else takes half the time for me, though. Getting the bare-bones functionality is only half the battle, usually.