Hi, my plug in is on 24/7 in my lounge, it keeps my golden retriever calm all the time, He has no reaction to the odd farmers gunshot or scary bin day collection which sound to him like thunder when they are rumbling back empty! On pet remedy website it states it lasts up to 8 weeks per 40ml I find it lasts longer, you can check when liquid is nearly used up, when I can’t see any more liquid in bottle to soak up wick I change it, you can use 4 x 40ml refill bottles to 1 plug xx