Make sure you bypass the resin and flush the membrane for a few minutes each time you start up the system. If you're worried about DI Resin usage, here's some tips. I'm going to try and explain what my triple TDS meter shows me, so maybe that will help you understand why your resin is being exhausted so quickly.
When you first start up the RO/DI, you want to bypass the Membrane to flush it of surface debris etc... Let this run for a minute or two. Next when you put pressure to the membrane the ratio doesn't immediately reach your target rejection rate, it takes a few minutes to reach that low output. My water is around 350ppm, during the membrane bypass (flush) I'm getting around 300ppm out of the "product" water. If you flipped the bypass off that 300ppm would be going to your DI Resin immediately. It takes about 2 minutes for my membrane to drop from 300ppm down to 5ppm. During this time I have a bypass on my DI Resin which just goes down the drain until I reach 5ppm. At that point I reengage the DI resin and capture the product water into my storage bins.
My suggestion, use 2 bypasses, one on the membrane to flush, and the second before the DI Resin. If you run each bypass for 2 minutes (a total of 2 steps for a total of 4 minutes), you will save a ton of DI resin in the long run. You can verify this yourself with a triple TDS meter (~45$), or take it as an act of faith. Personally I would have opted for the later, but nobody explained that to me when I bought everything!
