Since you have clear filter housings, the 1st cartridge is the sediment filter and it's white when new. When it turns brown it's time to change it. They are usually good for about 6 months, depending on the water quality going in and how much water is processed.
You can test the output water with a TDS meter to check if the RO membrane is good when you have new sediment and carbon block filters in place. They are usually good for about 2 yrs depending on how much water is processed and the quality of the water going in.