It depends on the yeast strain and where it came from. If I am growing yeast from dregs, I'll let it run for several days. If I'm stepping up a liquid yeast from my inventory, 24-36 hours, though I usually am not in much of a rush to get it off as long as I have 24 hours to crash the yeast before brew day.
I also grow extra large starters to harvest from of all of the yeast I use. If I am starting from a dry yeast packet, it tends to take longer than liquid yeast to ferment out the starter, so I'll allocate at least 48 hours.