[ZeGermanHam]

The general rule of thumb is every 2 years, but to be more specific it depends on the rate of pH degradation in your particular engine/radiator. If you want to keep track of things and change the coolant only when it actually needs to be changed rather than a predetermined schedule, buy some inexpensive coolant test strips: https://www.amazon.com/Cool-Trak-311.../dp/B000MIUJMO.

However, I'm not necessarily opposed to just changing it every 2 years regardless of pH. I think there is a benefit to making a routine habit of getting underneath your car and looking around for other potential leaks/issues while you're taking care of something else such as changing the coolant. Sometimes we need an excuse to get underneath and have a look-see.