reportyourlevel,
You have it easy! :-p
Over here (another UK regional) we have a minimum of once every 2 hours (but normally ends up between 1 and 1.5 hourly depending on the Duty Wildlife controllers preference), after traffic gaps longer than 20 minutes (and at dawn/dusk its after traffic gaps longer than 5 minutes) and ASAP after a state change to a wetter condition
And we have to keep that up H24/D7.
Done between movements (again no sterile runway but we aim for gaps which effectively give us a sterile runway), surface state reported to ATC on vacating and added to ATIS. State can only be changed on ATIS following Air Ops, ATC or RFFS inspections but if it starts pelting it down and we dont have time to inspect before next movement "Unofficial observation from the tower, runway wet, wet, wet" can sometimes be heard.