That depends on a few things, I have a few whisper power filters and the instructions says to replace the Bio Bag once a month, I dont use them unless I'm trying to rid the tank of meds. , tannins or cloudy water.
It also says it maybe necessary to replace the Bio Foam once a year. The foam rarly breaks down so you should'nt have to replace it for quite a while.
It basiclly comes down to population of your tank , how much do you feed and maintenance, what to clean, how often and how much.
I feed the recommended amount, my maintenance is this ( every Wednesday and Sunday I gravel vacuum, change 25% of the water each of the two days mentioned and with the old water I rinse the foam, so as not to harm the bacteria, I clean the tube and the impeller, maybe once a month I clean the filter box.
I have four tanks going right now(20 g long, 18 g tall, 10 g and a home made 2.5 g)(and soon to have another 20 g long and a 55 g up and running)but with the four tank going right now it takes me 2 1/2 hours Wed. and 2 1/2 Sun. So I spend 5 hours out of the 168 hours that are in a week cleaning four tanks(50.5 gallons of water) but with each water change and cleaning of the tank my fish are more exuberant and delight me with their child like playfull characteristics that it is well worth it....hope this helps... and hope you enjoy