Again, think it depends on how heavily stocked you are and the condition of the animals - our field was free for about 2 months last year and we had only 4 sheep in it until recently - the grass has really come on and it looks lush and very clean, so again depends on the weather, your land etc - don't think there's any set rules you have to go with your instinct. From what my vet said though, don't get paranoid about worming if you think they look well and your land is good. I think it's easy to over-worry, I know I do.