My sister asked me this & I thought I would get other opinions. She took her car to get the anti-freeze replaced because it had been 2 years. The mechanic said it tested down to -25 (NE Ohio) so didn't need replaced. I told her that testing doesn't show how the anti-rust and lubricating properties have broken down, but maybe anti-freeze is better than it used to be, and I would probably defer to a "professional" mechanic's visual inspection. Opinions?
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'86 244 160k, 88 245 185k, 90 245 153k, 90 745 waiting for me to fix

It is not just about the ability of the coolant to not freeze. Unless you have added water to the system to top it up the protection temperature should not change.

I am no expect but have been lead to believe that the additives that provide internal protection in the system as "used up(?)" in that two year service period. This protection may not have been as acute in the past with all cast iron material, but the addition of an aluminum head on a cast iron block brought a different level of chemistry into play. Having personally found electrolysis damage on two different heads on cars that have wandered in here I make it a habit to keep my coolant changed every two years.

Every other year I change my coolant and flush the brake systems on our cars; cheap insurance.

Randy

Prestone green: every two years
Volvo C with distilled water: every two years
Xerex G-05 with distilled water: every three years

It may be Old-School but I do mine every other year.
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'75 Jeep CJ5 345Hp ChevyPwrd, two motorcycles, '85 Pickup: The '89 Volvo is the newest vehicle I own. it wasn't Volvos safety , it was Longevity that sold me

Ditto same for me