It's a fairly new concept we started a few weeks ago and so far the requests have been reasonable. What I've been doing is setting them to run a week. However, if there aren't many new requests, I don't mind keeping it up another week or so.
Unless there is a request for a different banner. Then I would have to manually create that ad and run it for a week.
In short, generally once a week. Same ad with not many new requests, two or three weeks.
Hopefully that makes some sense