how often should a car battery be changed?

I don't know, I wouldn't change it until it stops working. I've had the same battery in my 2007 Jetta for almost 4+ years and I live in Colorado, where the winters are harsh and the summers are hot, and it has yet to fail me or give me any trouble

Battery health really has very little to do with expiry dates or scheduled replacement. The best preventative maintenance, in my opinion, would be to find a parts house with a good battery tester. A good battery tester electronically checks the overall condition of the cells. Depending on how the car is operated, and how good the charging system is, a battery can last for 5-7 years, maybe longer. I'd get a test done before winter hits, perhaps as part of a winter prep.

Car batteries should be kept charged all the time. It is charged when the car is running but if the car is not used it should be kept on a charger. The worst thing for a battery is to leave it discharged. If it goes flat often then it probably needs replacing.

If you're driving the car regularly the alternator will take care of it. However if it sits for long periods you might want to invest in what's called a "Battery Tender". That's just a trickle charger that you hook up when it's sitting for a long spell to keep the battery charged. Any parts store will have them.

Whenever it fails a test with this:

http://www.harborfreight.com/100-amp-6-volt-12-vol...

Probably when it doesn't work anymore, to save the money. Its a hassle when it dies but, it's worth the save if you have jump cables with you.

they can last up to like 6 years if used daily... just whenever you notice its not starting up as quickly