Changing Tranny fluid, what frequency have you found to be a good rule to follow for a 1989 302 V8?
I don't live in, or by or drive on dusty roads, just city and hwy travel and occasionally pulling my fishing boat.
I have thought it good to keep a eye on the color and smell of the fluid to determine when to change it.

 

Trans fluid will normally get darker with use, normal.
A lot depends on how and where the vehicle is driven, hot climate, hills etc.
Transmissions worst enemy is heat , that is why some folks operating under severe svc might install an aux cooler.
Some folks don't have it changed at all and they seem to get by,,,,,,,for a while.
Usually around every 30,000 mi is a good starting point.