I Clik the lambs as soon as I see a greenbottle, the main flystrike culprit around here, and put an entry in the diary to reapply 16 weeks later if the flies are still active. Ewes we just keep an eye on (shearers don't want to shear a Clik'd sheep!) and apply it as soon as the fleece is about 1cm long (enough for the maggots to cling to). Feet, of course, can be struck, so limpers need to be inspected promptly any time.