To date I have always drained and filled on a four month basis. I also use swirl away, towel dry and apply spa gloss, change N2 and micro filter and do a general maintenance. My water is always clear at four months, but I marvel at how nice fresh water is. Now, after watching my CYA levels get up over 100, I am considering doing a quick water change half way between my regular full maintenance water change (except mid winter). I am less concerned about TDS, but as water ages, it certainly gets a little harder to keep.