Everything on a car can die, break or rust. The alternator will fail one day, but I wouldn't worry about that for 8 years or 150k. It could go sooner, it could last longer. Its electro-mechanical, stuff happens. The nice part about the alternator dying (if it is charging till its death) is that you can run on the battery till you can get it home or to a shop. No lights, fan, radio, and it will last hours on a good charge. It will start more than a few times. If you need the lights, its going to drain much faster, but it should still go a while. If the bearing siezes (happened to me), you could be in more trouble. If the alternator stops turning, the belt could burn up and the car is done till you fix it. When my alternator siezed, I was able to remove the belt and drive home, but that was back when there where 2-3 belts on cars. You can't do that these days.

Batteries are good 4-6 years if you don't run them dead. Running them dead reduces storage capacity. You'll notice that the most on cold days when you need to crank the engine a lot.

 

All the above info is good however no one has told you about the dreaded 'dead cell'. At the very end of a batterys life, the normal sulphation of the cells ( chemical action) will cause one cell to short circuit. One morning when you crank the motor you'll get a grunt from the starter, all power will be gone=new battery required. Nobody can tell you when your battery will fail however....
1. Batterys last 3 years, if yours is older, youv'e got a good brand, else you're lucky. *** Makes and models will vary this period.
2. Buy a $ 2 hydrometer from an auto store to check the batterys 'state of charge' ( if the caps on the cells are removable).
3.If you are really dependant on the car, and the battery is 3 years + more, replace it!!

 

After reading this thread I put my battery on Ye Olde' multimeter. The voltage was reading 12.68, time for a replacement?

 

I have NEVER had a battery last less than 5 years in the 26 years I have been driving.Even in the worstChicago winters. Interstate, Delco, and Die Hard have always been good to me. Most good batteries come with a 60-72 month warranty. Interstate even has some warrantied for 85 months. The free replacement period is less than that (18 months?), but they will give you a pro-rated credit toward a new one if it fails under warranty.

I have replaced 3 batteries that I thought were dying. All of them gave warning signs. Usuallythey will not crank as hard or for a long time. The motor will usually start, but it sounds like its laboring.

Putting a voltmeter on a battery will tell you almost nothing. If you get a reading of 6 volts, obviously there is a problem. Reading 12-14 volts means nothing. You need to measure the current under a load (Starting).This requiresa nice meter with an amp probe. If you don't have one, take it to autozone or pep boys or some other parts store to have it tested. If you don't want to test it or ar that worried about it, go buy a new one. Piece of mind is probably worth the $75-$100 you'll pay for a good battery.