1970 original transmission
 

My girlfriend's grandmother's 1970 Camaro still has the original transmission.

Given its age, at how many miles --or how many years-- should it eventually be switched out?

She still runs the engine once a month to make sure the car still runs, but beyond that, it's been garaged for the past decade.

if its working leave it there just check fluid levels

Run it until it starts slipping, or not functioning properly. If it sits a lot, I'd start driving it around a bit once a month. Sitting is harder on a car than almost anything else. I'd consider a trans fluid change and filter, at least every couple years, even with so little use.