When to replace the air filter in 2000 Carmy?

Replace as needed. Buy a new Toyota air filter. Remove the old air filter, hold it up to the light and see how much restriction you observe. The manual recommends blowing in the reverse direction with compressed air, but when the light shows dark areas, just change the filter. Be sure the sealing surfaces are clean, and the filter is seated fully.

Air filters aren't that expensive.

I just as a routine, replace the air and cabin filters on both rigs yearly (about 12-13k miles) and usually they are fairly dirty. (Avalon and Highlander)