When you should change car manual transmission fluid?

When the fluid no longer look red. Transmission Fluid normally doesn't have to be changed that often.

For a manual transmission, maybe after 100K. The manual transmission fluid is thicker and hardly ever breaks down. Unless the gears are clashing, etc.

But for an automatic transmission: that would depend on how you drive you vehicle. A good idea for a transmission fluid change would be every 3rd to 4th motor oil change.

The transmission fluid is completely different from motor oil, but will eventually wear out.

I say a fluid and filter change. It is usually a bad idea to get a transmission flush if the transmission is doing good, so far.