good question. I have been researching this same issue. Seems to depend on some factors such as stocking levels, water quality, etc...
When cleaning your filters I think it is most important not to scrub off all the good stuff you need to keep the tank established. Rinse the parts in a bucket filled with aquarium water. This should help with keeping the beneficial bacteria on the filter.
I too have a Fluval 404 and 304 on my understocked 125g and I change out both the carbon media and ammonia media once a month. I also squeeze out the sponges inside a bucket of aquarium water when they get really dirty, about once every 3 months. The upper baskets contain bioglass rings and I never do anything to disrupt that. I am probably changing out the media too soon and too often but I have been doing it this way for over a year and feel that I'd rather be safe than sorry. I have noticed that the water seems slightly clearer at the end of the month than after I have all new media in the filters. It is my observation that when I change out the media I am losing some beneficial bacteria no matter how careful I try not too. Lets hope the guys who kow a lot more chime in. There is a lot of good advice on this forum.