Quick take
Use six months as a default only when the manufacturer does. Replace sooner if rated capacity is reached, flow falls or taste/odor returns.
Why six months is common
Many OEM refrigerator filters are rated for roughly 200–300 gallons and about six months of typical household use.
Heavy users may hit capacity sooner
Large families, frequent ice use and high daily water consumption can reach the gallon rating before six months.
Low-use households
Even with low volume, manufacturers may still specify a time limit because stagnant cartridges and media age matter.
Compatible filters
Aftermarket cartridges can use different capacity and certification claims, so do not automatically apply the OEM replacement schedule.