I work for a filtration laboratory and spend 50+ hours a week conducting tests on HEPA filters and wanna-be HEPA filters. Depending on how big your filter is, it should last a very very very long time, for as little air as it sees. As long as you are getting air through it, it is fine. Actually, the more "dirty" a filter becomes, the efficiency of the filter media improves. So as long as it stays dry and doesn't get moldy, don't replace unless it is completely plugged up and you can't get air to go through it.