I use them in between grows. After I harvest...I put all my pots, tools etc in the grow room - hang two no pest strips - turn off all the ventilation and shut the door for 1 week. I've never had a pest problem since I started doing this. But the strips do not work if the room is ventilated. They are mighty powerful and give me a headache if I don't air out the room before going in to work.
I think 1 is good for a 10x10' room, if not bigger. I put 1 in my 2x2' tent for a few hours with fans off and vents shut. Completely seal off where ever it is that the strip is in, no air flow for at least a few hours.