Generally, the battery is good for 3 to 5 years.

After it fails a load test during a semi-annual service, it is time for a new one, even though it is still more or less serviceable for up to 6 months.
I have found the battery dealers to be too lazy to punch the date of sale on the battery, so this I do myself, as well as the terminal and connection service..

I typically get about 8 years out of a battery, and I live in Minnesota where it can get very cold, which can be hard on a battery. But I do a few things to make sure my battery has a good charge before I shut off the car and I think that helps a lot. The things I do are: Turn off the heat or air conditioning a few blocks from home. Turn off the headlights before I back into my garage. Both of these things help to get just a little more charge into the battery before I shut down the car. Some people leave everything turned on and even leave their headlights on, letting the automatic shutoff do its job. I think that makes your battery work harder. Now, I have no scientific proof that what I do works, but I have used this technique with Hondas, Subarus, Chyslers, and Toyotas and I have never replaced a battery in a 5 year span of a vehicle. I once had a battery last 11 years.

Over and Under, without doubt you are doing all the right things.
Of primary importance is using the correct engine oil and maintaining a high level of engine tune.

I'll bet that you do not replace starter motors either.. Never have I had to, but I did "rebuild" the one in the '50 Buick Super 8...

I think it is the excessive heat that harms batteries , rather than the cold....
Also, you must be using the top of the line units, not the 18 month cheapie - which can be good for 9 months !...Ask not how I know this..

If you're parking the car in a garage, if the garage has electricity, why not get a battery charger/maintainer and connect it to a cigarette lighter plug so you can keep the battery charged up?

This is what I do when I leave my car sitting in my garage for a week or more. (Only works if your cigarette lighter is live without the key on).

When it's parked outside and I let it sit, I use one of those solar chargers that plugs into the cigarette lighter.

Thus far, then, Brian, what do you think of this solar charger ?
Is it worth the money and reliable for those who let a car sit for a week ??
To me it seems to be an idea worth considering.A battery does last longer when always fully charged, and the more gentle the charge, the better..

Saab came up with this idea(built in solar chargers) 20 years ago, along with a cabin fan to cool the interior during these sitting times.

I think it is. My car has a 25mA current draw when sitting idle (that might be high, I don't know..it's got a bit of add-on equipment). So even before we even consider self-discharge, that means the battery has lost 4.2 amp-hours of it's capacity sitting for a week. Self-discharge estimates vary, but it goes up with the temperature. I figured that with self-discharge, I might be losing 8 amp-hours of capacity by letting it sit for a week. I guesstimate that the battery is about a 52 amp-hour capacity battery. (Group size 58 if I recall correctly--whatever fits in a 1988 Mustang 5.0--the original factory battery is specified as a 52-amp hour battery--I would expect that the Diehard replacement is about the same capacity)

Allowing a car battery to discharge below about 10% reduces it's lifespan. In this case, that would be 5.2 amp-hours.

The solar charger I used is a 3.2 watt charger from VW. VW ships every car they make with one of these chargers to keep the battery from going dead. I got two of them off Ebay for $40 shipped. (Do a search for VW solar charger on Ebay to find them). They plug in the cigarette lighter. They're probably the best charger out there for this type of thing.

I hooked one up and tested it--even without being in direct sunlight, it was putting about 50mA into the battery. I didn't test it in direct sunlight (mainly because the clouds weren't cooperating, along with the angle of the space I was parked in), but the label says it puts out 170mA.

Very interesting,Brian, thanks for the info.
I wonder if the VW forum people know about this...

Maybe, sometimes the VW dealer keeps the chargers and when they get too many I assume they put 'em on Ebay instead of trashing them.

A fifty dollar item being pitched - even the idea ??
Unbelievable !!
No wonder VW is hurting !
Now, if they were smart, the solar charger would be included with the car..
Of course, if I were the VW prep man, I would sell them on eBay - if I could, with permission...

Ive alwyas thought that Turning off the a/c and radio, before getting out of the car will cause premature wear on those components. Im not really sure if it does but thats why I dont do it.

I can buy a $28 battery and add water 2 times a year and it will last quite awhile. I figure maybe i can get 5 years out of cheapie without turning off any equipment.

Some canny VW shoppers insist that the dealer toss in the solar charger as part of the deal. Other VW owners make -- um, friends -- with folks in the prep area and can get them that way. I suspect they're tossed because VW does not want to be responsible for supporting/warranting the part ....

earthworm - yup - never replaced or rebuilt a starter yet. And no, I don't buy cheap batteries. I usually opt for one that offers 60 to 72 months and plenty of cold-cranking amps.

Living in a cold climate (at least part of the year) makes one be careful. Coming out of work to a vehicle in the dead of winter that won't start is an inconvenience. And it can also become an issue of personal safety. Not only from the elements, but from dangerous people as well. I took a self-defense class back in college. I'm a big guy, but I thought it would be a great time as the male/female ratio would be much to my advantage (and I was right). I learned a lot of good information. One cold day the instructor asked how many people could not get their car started because of the cold. For those that could not, he told them they needed to do something about it, as they were putting themselves in harm's way by not maintaining transportation that could get them out of a bad situation. This self-defense class emphasised keeping out of trouble even more than getting out of trouble. I now view keeping my vehicles running well as a matter of self-preservation.

GoodOleBoy - I usually own a vehicle 10 years or more. My turning off the switches to the air, fan, etc. must be okay, because I have never had to replace any of the switches. I, too, had heard the argument that it would cause wear and premature failure to the components, but I have yet to experience that to be the case.