There's a CAA pink sheet about choppers. It's time for rotor wash to disperse that is the important thing rather than distance, in the same way as you wouldn't take off or approach straight after a big jet. Been a few accidents where light aircraft have been rendered uncontrollable by rotor wash, for some reason that I'm not brainy enough to understand rotor wash hangs around for a long time near the ground after a taxiing chopper has gone past. I always give them a wide berth. I don't like to be too close to them anyway as they are so ugly they render me unconscious.