it lasts for over a month... the way i have it set it runs for half hour on/off durring the light cycle... its a sloppy setup for sure but it works... my ppm goes over 2000 ppm while its on then by time its about to come on again it has dropped down to 700 or so.... when i have my short range timer hooked up it goes on/off every 15 mins and it stays much more constant