Don't think all yeast are the same, some are done at 18 hrs while others take 30 hrs, especially any of the English varieties. Yes, stir continuously on the stir plate. Ambient temperature has a lot to do with times. Some stirrers actually generate heat which is transferred to the flask. Since at higher temps the yeast can produce yukky flavors, that's why we decant the initial starter, and later add cooled pitching temp wort to restart it. If you built a DIY starter plate make sure there are cooling holes somewhere because some motors produce a lot of heat.