I did mine at about 4 years and maybe 500 hrs. I suppose it's due again since it's been another 3-4 years. I figure it's good to change once in a while to replenish the corrosion inhibition chemicals. I do try to be very "clean" with the coolant and use good stuff. When I change, I do a chemical clean/flush, rinse VERY thoroughly with tap water (lots of calcium), drain VERY thoroughly, and then refill with distilled water and "FleetGuard". All good to date.