I know a lot of factors can play into it, but in general, how long should they last?

Just past the four year mark, the one I have died today. It didn't see regular use this year like previous years, but I was putting a slow charge on it when not being used (driven) to keep it up.

I'm guessing the lack of use and charging shortened its life. (The charging system in the vehicle is working correctly.)

This is my first experience with one of these batteries.