I drive a 2016 Scion FRS, and based on the maintenance schedule of the coolant:
1st replacement interval is 11 years/137,500 miles (220,000 km). 2nd replacement interval is 6 years/ 75,000 miles (120,000 km) after the 1st.
...I don't really understand the point of replacing the coolant. Why would it need to get replaced if you're constantly replenishing it? Especially during a long period of 11 straight years, wouldn't it have been replaced several times over?
I said "constantly replenishing". Constant could be any frequency. The definition of constant means occurring continuously over a period of time. So let me be specific, "topping off when needed". Also my car is still relatively brand new, under 10k miles. There aren't any signs of leaks that I know of, so it's doubtful I have a leak.
I already answered my own question prior to looking at any of the answers here, but correct me if I am wrong. Basically, by "topping off", I am diluting the ratio of impurities to non-impurities in the coolant solution, but I am never removing the impurities themselves. When coolant gets used, it evaporates, leaving the impurities behind, so replacing the entire supply of coolant is necessary as a method to carry away the impurities. The only person who came close to this answer is user33191.