Up to 30ppm is fine I think, personally I change mine at around 15/20.
These peeps that change at 5 and who's that, that's changing there pre filters once a week WHY !!!! Wasting ya money. Personally every 3 to 6 months. Filter changing. Membranes 3 to 5 years depending how slow my tank is in filling, then it's time to change membranes 👍