How much time will depend on cost. We need to get some price estimates before we can make a reasonable decision. I think we want to maximize viewer-minutes (audience * air time).
Saturday is normally the worst TV day of the week. So maybe Sunday, when people tend to be home from work. Close to Super Tuesday would be good, but Not SuperBowl Sunday -- we don't want to compete with that. We also want the campaign to have time to be able to put the money raised to good use. Maybe a couple of weeks before?