I always set it to 2 days before I leave, so I can check and make sure nothing is being delivered. Then it usually takes at least 7 week days before mail starts showing up at the other end.
One thing I've never figured out is when the P.O. stops forwarding in relation to when I told them to stop. It's almost like they back up their actual stop date so that nothing arrives after I've headed back home. Maybe that's giving them more credit than I should...