WELCOME TO AA!!
You shouldn't have to change the "cylinders', that is where your beneficial bacteria reside. The carbon, once a month, that just keeps your water clear. You may want to rinse out the sponges in old SW
, when you do your PWCs. I only did mine once a month, but I have a low bioload. How many sponges are in that filter? I had 4 in my 304.