Maybe it's because I always bought Sears batteries, but I always replaced them at around 14 - 16 months. I never had one last 2 years. On the other hand, they're relatively cheap, so I guess I came out about even.
This was on an 86 fxrs, btw.
I'm now looking at replacing the battery on my 03 rg. As far as I know, it's the original. It's getting hard to start if I don't ride it every day, and while I might coax a bit more out of it with a tender, 5+ years is pretty much at the far end of things, and I don't want to get caught somewhere. It's not like the old bike, which could always be pushed a bit to get it going.