What is average lifespan of a battery?

[DaveNW]

My '02 S had an Interstate battery installed in 2013. Over the last few months it's been losing charge more and more, and the Battery Tender I use became more of a necessity. A friend was storing the car for me the last month or so, while I did some major home improvement stuff. I didn't know he didn't have a Battery Tender of his own, so the car sat without anything helping keep it charged up.

Yesterday when I went to collect the car, it wouldn't start. We jumped it, and it started normally, but after turning it off an hour or so later, it wouldn't start again. Thinking the battery was just extra-low, we left it on a charger all night. Today, same issue - no juice. So now I have a brand new battery, which I will monitor the performance of.

The question for you experts is this: What's the average lifespan of a battery? I live in the Pacific Northwest, where temperatures rarely get too hot or cold, and batteries in my other vehicles seem to never fail.

What's your experience?

Dave