Our manual says that we stay on westerly ops until we get a tailwind of 5 kts due to westerlies being preferred for noise reasons. This is conditional on it being a dry runway and a minimum crosswind component of 12kts.
Other factors we take into account:
-Work in progress, if it affects any runway exits, departure holding points.
-Lighting faults; at night, if the taxiway and runway lighting system has a fault on or near a runway.
-Navaids
-Traffic and forecast wind (ie. if it's 0545 - 15 minutes before the inbound stampede begins - and it's 180/5kts with westerlies, and at 0700 it's forecast to go to 090/10kts, you'd swap ends then because it will cause a whole lot of grief and delays at 0700!)
As for your first question, I have no idea, but anecdotally I reckon a week or so.