[M.D. Holloway]

Thats a great question - the only answer is "when it needs to be changed'. I know that sounds flip, its not intended to be. Coolant or any fluid for that matter will undergo many chemical changes and become contaminanted over time. The rate and degree is different in every case. The only way ever to know is to test it. Fortunitily testing fluids is very easy and done so either for free or on the cheap at any parts store or garage.

For coolants, the balence between water and glycol is important is is the ability for the coolant to continue to passivate a metal surface. Keep in mind:
If the dilution rate was high, then the following could be experienced:
• Poor heat transfer
• Loss of rust protection
• Excessive coolant costs

If the dilution rate was low, then the following could be experienced:
• Excessive pump wear
• Loss of rust protection

If the pH was high, then the following could be experienced:
• Corrosion

If the Water Hardness was high, then the following could be experienced:
• Deposits

If the Water Hardness was low, then the following could be experienced:
• Foaming
• Metal Oxidation