The carbon filters in this machine are fairly sparsely filled and don't have a significant amount of carbon in them. Typically replacement is recommended after about 6 months or so, but I've found that with heavy use, similar filters last only a month or so before losing effectiveness. It has to do with how much odor and VOC is present in the area you are using the filter in, as well as how much it is running.

I made my own filters with over 2-3x the carbon by weight (bought from Amazon) in them, and they last about 2-3 months before they start emitting an odd smell (but they still seem to reduce odor, but they are no doubt trading new smells for older ones as they fill up).

I usually change the filters as soon as the smell becomes odd, in my case, the smell is like a slightly sour and damp version of the fresh and dry chocolatey smell that they sometimes have when new.