How long until you should change the engine coolant in a vehicle?

i hear the coolant turns to acid but how long does this take.How long until you should change the engine coolant in a vehicle?conventional anti-freeze should be changed every 2 yrs or 24k miles. longlife I would not stretch past 5 yrs or 50k miles. avoid adding tap water as it will shorten the life drastically.How long until you should change the engine coolant in a vehicle?eveery 5 years is goodHow long until you should change the engine coolant in a vehicle?every 2-3 years