The legal minimum is whatever it says in your ops manual.
There is no laid down rule, but this is one of the items that the CAA will look at closely before issuing your AOC.
It used to be not unusual to see just a requirement for a good briefing, but recently they seem to have tightened up on that and cat C requires training and/or a visit. Some operators get round this by then recategorising all the airfields as cat B..
