08 May 09 12:16 am
Track and Test. I have a campaign i am running right now that converts at about at about 3 percent Tue-Thurs however Friday-Monday it converts at way less than 1 percent. So to keep my ROI where i want it I pause the Campaign ever Thursday night at 9pm and un-pause it Tuesday morning.
You will just have to track and see. There is no magic number or time.