On my Honeywell filter, the charcoal filter is the pre-filter and is
the only one that has to be replaced. Honeywell says that their HEPA
is a "lifetime" filter which never has to be replaced so long as the
pre-filter is used and changed regularly. The pre filter cost about 7
bucks. I'm not sure how often I replace it, but not very often. There
a light which which comes one to tell you when it needs replacement
and by the time that light comes on it is full of stuff.