Just got a juwel trigon today when setting up was reading filter manual because it has a whole bunch of levels and wanted to put them on right order, when looking at the leaflet I saw that the poly filter part needs changing weekly carbon monthly I think the blue standard sponge needs changing every 8 months there is a sponge called nitrix I think it's already got some bacteria in it that cleans nitrates so less water changes. And the bio- part needs changing every year.
Now I think it all depends on 1) filter media size by tank size 2) amount of fish and any other waste makers 3) as mentioned flow rate
The juwel will want you to buy their filters daily if they could so i probably won't be changing them as often as they recommend.
I'd get rid of your filter, and get a better one. (please dont take offence)
your filter has no proper mechanical filtration (one strip of white pad does not count)
but till then:
you will find washing the white pad will bring it back to life. (at least for a few goes)
junk the carbon (no basic internal or external filter for 15 years has needed carbon) this will give you more bio area, if you replace it with plain sponge.
junk the "nitrix" (lol what a con that is) again you will gain bio area if you replace it with plain sponge.
your sponge media will only, thats ONLY, need replacing when it starts falling apart.
Carbon, Nitrix and forced changing of sponges are just methods of trying, by Juwel, to get money for no reason. I would say its a con. (as would anybody else with long experience of filtration.)
I'd replace the heater too, before it cooks or chills your fish.