I'm such a lazy ass:
- 20% monthly-ish (4 to 6 weeks)

- FW (w/kalk) top-off

15% every two weeks.
RO/DI Fresh water for me. Every two weeks I let my RO/DI unit run overnight (which gives me enough to pump out my 15% needed for salt mixture and the rest stays for ATO). 100 gallon tank.

7.5 to 15 % weekly.
top off with ro water

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12.5% bi-weekly (10 gallons on an 80 gallon system)
RODI ATO.

PS. Every other month I do a larger water change with a detritus clean up. It's usually about 25-30% in one shot.