Even in the low teens, a refill should last for a lot of water. How many gallons have you made on that cartridge?
Also, think about adding a bypass for the product water to the drain. It takes some time for the membrane to reach its full performance when it is pressurized. You can see the spike on your TDS meter. It can sometimes take a minute or two to go from the tap TDS down to the normal level. Each time you run this through your DI, it uses up more resin than probably dozens of gallons of water at regular operation. On mine, I have a valve to dump the product water, I turn it on and wait a minute or two, then I turn on the valve to the DI and shut off the bypass. The resin lasts much longer.
At my last place, incoming TDS was 800-900 so I started finding tricks like that because I was burning through a lot of resin and it was getting expensive.