same with me on the color changing DI. I just got it and prob made about 90/100 gallons and it's half way through. My water is pretty bad though. Prior to RO it's at 700ppm TDS and after RO it's at 30... Other people seem to have their membrane get them to about 5/10ppm... So I assume it's from the bad water to start with.
I do find that it's always the first 10 gallons that gets me on the DI... Once it's going, it doesn't seem to change as fast. Not sure though, I've only had it a little less than two weeks. But I made about 40 gallons one time, 20 gallons another and then 40 on again. So all decent sized batches.
I'm hoping I don't have to get a new DI cartridge every 200-300 gallons.