Actually, I've just read the Fly Guard packet and it says one application is effective for up to 3 months. It's a 75ml bottle and costs around £10. Rearguard should be applied every 10 weeks, is a 25ml bottle and costs around £20-£25. Bear in mind that if your bunny is hopping around on damp grass or gets wet, some of the product will be washed off and its effectiveness lost.