I’m not familiar with cartridge filters (got one, never used it). Are you able to stuff a new cartridge in there while still retaining the old one? (To allow bacteria to form on the new one before removing the old)
Like Aiken said, the problem with cartridge filters is you throw out a vast majority of the bacteria that is your cycle every time you change the filter. Yes you still have some bacteria that remains in the rest of the filter and it’s likely not an issue for most people if you aren’t heavily stocked. Ideally we would all use reusable media that is maintained on a staggered schedule to reduce impact on the cycle, but that’s not always practical.
Since you’re using the pre filter sponge and it’s got the blue gizmo that’ll house some bacteria, it probably isn’t ever an issue to continue using the cartridges. Just make sure you leave some time between the maintenance of the pre filter sponge and anything else when you do change the cartridge. As you likely know, a full clean of the filter, pre filter and a new cartridge has basically 100% chance of crashing the cycle
As for when to change the cartridge. I know from experience a fine mesh pre filter sponge makes a huge difference on the maintenance I need to do on my comet tank. It literally doubled the time between cleanings of the filter, and only requires a quick rinse of the pre filter sponge once a week now. If the cartridge isn’t restricting flow, I would personally run it until you start to see a noticeable increase in restriction. That’s how I maintain mine anyways, when the flow is reduced, it’s time to service it!