when to change battery?
 

Any recommendation to a safe battery life e.g. for a 2003 V6 4x4 4runner? Hate to get stuck during off-road, or in the winter with a dead battery after a day of skiing. Should they be replaced every 3 years, or is there a way to monitor the voltage and replace it if it starts to drop below some value?

My car's original battery died after 3 years, but the replacement is working its 6th year without any problem.

You can take it to Sears, Advance, Autozone, etc and they have a tester that puts it under load and tells you the condition of the battery and alternator.

Or get a AAA membership and don't worry about it...

if it fails a load test then replace it

This is what I use.....most of the time you'll get some warning before it goes completely (not always).
You need to load test it to verify a good battery.

Personally, I'm 100% sold on AC Delco batteries from my local GM dealer to the tune of $70. They're sealed batteries/maintenance free and have their gas vents directed to the side to prevent terminal corrosion. They fit perfect every single time even in my Toyotas and I've never seen one spontaneously fail. Buying a good brand is half the battle. I believe that to be a good battery it must fit good too. None of those adapters or universal pieces of crap with the terminals in the middle that make you stretch your battery cables. A well secured $35 battery with the terminals in the right place will absolutely outlast a $120 battery that doesn't fit right and vibrates around the battery tray and yanks your ground cable off the interior fender well.

A battery can fail at any time. Most of the time, however, you can tell if it's degrading if you pay attention (and if you're lucky). Cranking speed on cold mornings is probably the most blatent indication. Another is how dim lights get when the engine is cranking or when you're sitting at idle with other heavy loads turned on.

Todays batteries have improved longevity compared to those made 10 years ago. Most decent batteries will last 4-5 years, easy. But there are a lot of variables and you simply need to watch for signs. If you're uncomfortable, replace the battery every 3-5 years and don't worry about it. Batteries are cheap.