First, a stirplate is highly recommended. There is a huge bump-up in yeast growth when you use it. Don't be afraid to build your own either - you can get all the parts from Radio Shack for $30-50 (I used these
instructions when I built mine).
If I recall, Jamil and John said during the
Brew Strong - Yeast Starter show that maximal yeast cell density occurs between 18-24 hours for a starter on a stir plate. So I'm with the previous posters - I try to make my starters the day before the brew session.
The only exception to the above would be if you want to step up yeast in multiple steps and need to decant off the spent wort between steps (which is sometimes needed for lagers or if your growing yeast out of a bottle). In these cases, you should really let the ferment out to completion (2-4 days), then put it in the fridge for 24 hours to help floc the yeast out, decant, and then pour the fresh wort for the next step-up on top of the remaining yeast slurry.
Hope this helps.
-Okt
