100k or 60 months, whichever comes first
He's 4 years in on the coolant that's only good for 5 years. "Time" matters more for the coolant than mileage. Even sitting there in the garage it's deteriorating.
A drain & fill is easy and keeps the system fresh. Especially with this HOAT stuff and aluminum parts its cheap insurance... i do mine every 2 years and don't have to worry about doing a complete flush which takes more work.
Corey it's not expensive at all. 2 quarts of tcase fluid and a gallon of HOAT antifreeze concentrate will be ~$50-$60.