Hi cuckoo I start with a fec and depending on the results and time of year ie spring rise , I may do it more frequently depending on the forecast but usually rely on vet advise re follow up depending on result and if there are lambs around. The adults can cope with a reasonable burden far better than young lambs.
I had wes done at lambing when the egg drop is usually higher due to the stress they are under, all neg but within 2weeks one ewe had a 400pergm count so it can vary and quickly, this was in April.
Glad to hear there are others around who make good use of some very good vets !!!!