I'm pretty sure the service schedule for replacing the transmission filter for a 94-96 B-Body with a 4L60E automatic transmission is an interval in miles only. I don't have the number handy at the moment, and a quick search didn't turn up a definitive number. I'll update later with the exact number from the FSM.
Assuming the vehicle is a Garage Queen that gets very few miles put on it, should the fluid and / or filter be changed on a time interval instead? If so, I suspect the filter would be good longer than the fluid. For a filter with maybe 15,000 miles on it, would it be ok to change the fluid without dropping the pan and changing the filter? Via incremental pumping out of one of the cooler lines?
This application does have a an oil-to-air transmission cooler. Does that have any impact on the life of the fluid? Especially if the vehicle is driven long enough to boil moisture out of the fluid occasionally?