Generually speaking, you should spin the yeast until either:
1) It's at high krausen (~18 hrs or when foam is at it's highest) and you're ready to pitch the entire contents directly from the stir plate.
2) It's finished the stir plant (24-48 hrs) and been crash cooled for a day or two.
The vortex isn't as important as keeping the yeast suspended and not throwing the stirbar, but, yes, the stir plate will be running the entire time (18-48 hours).
:: St. Pug ::