Water softener before RO/DI??

I have a 75gpd 4-stage RO/DI setup. It makes about 100 gallons of treated water per DI cartridge. the filters are about a year old but I have only made about 500 gallons of treated water in that time. I have ordered new filters and just got a TDS meter today.



My questions are:

With county water @ 210-220ppm TDS



(1) How long should a DI cartridge last?



(2) Is a water softener worth the $$ with TDS @ the tap near 220ppm?



(3) Is filter maintenance based strictly on elapsed time or is volume treated relevant?



Thanks in advance.