Hard to say when or how often. Bucks have been reported hitting them at all different times, there's no key pattern to scrape activities, in some areas they may hit them on a daily basis in other areas it may be every 2 or enev every 3 days, it's really anybogys guss as to when.
I think a lot of it is based on Doe activity and the phase of the rut, the best advice is to keep an eye on them from a distance and with a little luck they will check them during your watch.
Democracy is two wolves and a lamb voting on what to have for lunch.
Liberty is a well-armed lamb contesting the outcome of the vote.