How long carts last is hard to say..
But at Brooks pace they should last a year or more...
You need a pressure gauge,volumiser or tds meter to know exactly when the cartridge is clogged or spent.
Pressure should rise if filters are clogged,I don't like this method but do have pressure gauge built in on my unit...
A volumiser will measure the water in various ways in different stages.
FM-2: Filter Monitor with Volumizer - HM Digital - Bulk Reef Supply
And finally a TDS meter as the tds will rise as the filters become less effective.
1-3 years with 2-3 being more normal for the Thin Film Composite Membrane(TFC)
Here is a kit with an extra flush valve(we never talked about flush valves..)
BRS Universal to Universal Plus Upgrade Kit - Bulk Reef Supply