You should replace the Brita LONGLAST filter every 120 gallons, which is about 6 months for an average family. The standard white filter needs changing every 40 gallons, or about 2 months.
The filter change indicator light is your best guide. You can check the status anytime by pressing the STATUS button for 1 second. A green light means good, yellow means getting low, red means replace now. The system counts fills, so for it to be accurate, you need to always fill to the max line and reset it when you install a new filter.
If that didn't work because your indicator isn't on or seems off, you might need to change it based on time or taste. You may need to change your filter more or less often than the average, depending on how much water your household actually drinks. If the water starts to taste off or flow slower, it's time for a new filter regardless of the light.
Heads up: Filter performance is not certified after the stated filter life (120 gallons for LONGLAST). Changing it regularly is essential for it to work as advertised.