Quick take
Many premium shower filters use roughly 90-day replacement schedules, but capacity and actual shower usage matter more than the calendar alone.
Jolie interval
Jolie recommends replacement approximately every 90 days.
Sproos interval
Sproos also uses a recurring 90-day cartridge model across its filtered shower products.
Weddell interval
Weddell emphasizes rated gallon capacity and model-specific performance, which can be a more useful comparison than a simple month count.
Heavy use changes the math
Multiple people, long showers and higher contaminant loading can reach cartridge capacity earlier.