You only need to replace them after continuous use that would equal out to three grows.. I heard something like 12 months of use you should replace it.. save yourself some money and only attach it to your exaust when the plants start to stink, (when they are getting ready to flower/ flowering) and you can extend the life of your filter by 50%...
Although letting it sit , gravity could make the charcoal settle but who knows, im no expert..