They cover about 600 square feet. You need the Feliway brand diffuser and the refills. I have three of them plugged in at all times. I've seen a dramatic decrease in the aggression towards Ivy and in the amount of inappropriate peeing. I can always tell when the one in the kitchen is low--Onyx pees on the counter! They last about a month.

I order mine from EntirelyPets.com. It's an American site, but even with shipping and the exchange rate, it's cheaper than buying it from my vet(who wants close to 100 dollars per diffuser!)