Here's a related follow-on question--Has anyone ever had a 9V battery leak?

Perhaps the 6-month rule came about at a time when batteries were less reliable, and 6-month replacements made more sense? I've never seen a leaking 9V, and with the availability of lithium 9V cells now, you could go many years without changing those puppies.

I have fond memories of a 9V-powered transistor radio from Radio Shack when I was a child. It came in many colors, I recall. They don't make 9V radios anymore, it seems, but that would've been a good way to use up those cells from the smoke detector.