I think you are amongst the lucky few. Getting 8 months of use out of a DI cartridge sounds like a dream come true. By your math, you are getting near to 1000 gallons out of a single cartridge. That's pretty good.
I've only recently got my RO/DI system tuned in, so I can't say for sure how long the resin will last. I'd guess around 800+ gallons of water, but that's a pretty wild guess.
Its really more a question of how efficiently your RO membrane is opperating. If the TDS coming out of your membrane is 1ppm, your DI resin will last 5 times longer than if it is 5ppm.
The trick is to flush the membrane every time the system starts up and to allow the membrane to flush until it reaches an optimal TDS output. Any kind of start / start operation tends to be really bad for the membranes. Long periods without any use are also bad.
I run my top off RO/DI once a day to fill a resevoir. When the system starts up, it automatically flushes for 3 minutes via an electronic solenoid that bypasses the flow restrictor. An electronic float switch closes a second electronic solenoid as soon as the resevoir is full. There is a mechanical shutoff as a backup to the electronic system.
After a lot of trial and error, everything seems to be working perfectly. The RO membrane consistently produce water with 0-1 TDS. I expect my DI resin to last a long time.
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