I tend to get between 4 and 5 years out of car batteries. If you never want to be stranded and don't mind replacing something that isn't fully worn out, you could just buy a new one every four years.
In terms of predicting when the battery is about to fail, I have a small DC voltmeter plugged into the cigarette lighter socket of my SD. The meter scale is color coded to make it simple to see "Good, Weak, Poor." I think I paid about $10 for it at a camera shop that sells semi-pro video 12 volt battery packs. It's most informative before starting the engine, letting you read the voltage output of the battery prior to the alternator sending 14 or so volts, as well as seeing how much the needle drops when the starter pulls the voltage down more. My SD will start if the battery is around 11 volts. (The battery is fine, I need a voltage regulator. )
Hope this helps.
Robert W. Roe
1983 300SD 264K mi
[This message has been edited by Robert W. Roe (edited 07-19-1999).]