I'm pretty sure most replace the resin when measuring anything greater than zero. I look into replacing things upstream of the resin if the water going into the resin is higher than 10 so it doesn't burn too quickly. My resin lasts me ages.
22 out of your membrane is going to use resin fairly quickly. A few thorough backflushes may get your membrane readins down lower. I think it's a safe bet that your prefilters need replacing if they've been in use longer than 6 months.
The key to longevity for me has been religiously backflushing the membrane before and after use for 2 or 3 minutes.
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