Since you only have 1 filter, i would recommend cleaning it every 3-6 months. It really depends on how messy/stocked your tank is. Do you have a sponge filter? I've never liked only having one filter in a tank, in case one gets nuked. So, when i had tanks, i always had two filters, and i would alternate cleaning by a few months. That worked well. If you ever see your water decrease significantly from your outlet, that should be your indicator and should mark a time frame for you on how often you need to clean.