I answered my question by unscrewing the cover off one of my bodies using the eraser-end-of-a-pencil method. I bought this body 5 years ago from KEH so I don't know when the batttery was changed last but I checked the voltage and it was 3+ volts, i.e., original voltage. I guess as long as you don't let the main AA batteries run down and draw on the reserve battery it lasts almost indefinitely. I will plan on replacing it every 5years just to be safe.

I need snap ring pliers to get the cover off my other body.

Thanks all for the info in this thread. It's a great camera and I plan to use it for a long time.