A month later...

[TomPaine]

Yeah, I reckon 5-7 must be about average on a 9-5. I replace at 5 cos I'm risk-averse and since they always let go without warning at the most inconvenient times, generally midwinter!

[MikeIOW]

I've owned 3 Saabs each for 3-5 years, & I have only had to change a battery once.

On the other hand, our (mostly garaged & sadly rarely driven!) Nissan Figaro has been through 3 in 12 years.........after the second, I use an "Oxford Maximiser" to help maintain it, & that seems to help.   I would think a car regularly used on more than just a 'short' journey should have a good battery for some time....

[Audax]

4-7 years seems to be the most common age when replaced, there's a reason most battery warranties are 3 years....