Road marching occurrences

On average (not in a singular wing) how often does blue, bronze badge etc road marching occur as a whole?

This will all come down to your Wing Road Marching Officer, especially for blue and bronze as these are 1 and 2 day events usually done on a weekend.
Silver usually depends on when the big road marching events. A common event I have seen people silver with is WARMA events however that will depend on your availability to attend and distance from the event location.
Hope that helps.