This depends on what kind of fluid you’re using. If it’s an…
ACT fluid, replace every two years or 30,000 miles
OAT fluid, replace every five years or 150,000 miles
HOAT fluid, replace every five years or 150,000 miles
Non-toxic fluid, replace every three years or 36.000 miles
Of course, these are just ballpark estimates. Each container of the product advises you on how often to replace it, though.
Alternatively, you can check the color of the fluid when you flush it. If it looks brownish, it’s time to replace it.