the instant you purchase the laptop !!! … =P

lptp batteries die pretty shortly from what I've seen. AND they charge an arm and a leg for one, because its “designed” that way.

I'd state once it can't hold a charge longer than 2 hours ……..its time to get a new one………… basically this happens after 1 - 1 1/2 years of use. — or abuse.

buy it when you've got the money. you don't want it to surprise you when you've don't got it.

some GREAT tips to save then next one..
I don't know if #1 deserves #1 but hey, why not?
http://www.friedbeef.com/top-15-ways-to-…

Answer:
When you buy your new laptop, you'd see from the specs the amount of time you can use your laptop independent of the charger. This usually is around 3-4 hours. If it happens not to last less than 2hours then you might need to buy a replacement. A battery that doesn't last that long is a horrible companion on the go.

If you happen to use a cooler and still the battery package of your laptop can still boil some eggs then you must be looking for a replacement. Another way is to see if the charger also brings itself to a boil. A defective or aged battery can stress the charger bringing it to such temperature.

Another way would be to see your computer's performance in cd or dvd burning activities or media concerting tasks. If you happen to have a slow performance then this can minorly be attributed to the battery though this function test can be so subjective to a computer's specs.

If you happen to have a voltmeter (a digital multimeter as called by some) but this would rarely be the case, you can measure the output voltage of your fully charged battery. If it drops below 90% or hikes beyond 110% rated voltage then you might need to replace it.

Batteries should be replaced if they are more than a year since this is only their effective lifetime. You can save abttery life if you can have an alternate with it. Alternating can be done in weeks so as not to do much hassle. You can also prolong your battery life by literally not using it. Some laptops can be used like desktops where they have the ability to operate mainly on chargers alone. However, if a power interruption occurs so goes all those unsaved data.

Answer:
well if your laptop compuer battery perfomence is becoming low and the battery life time is becoming low compared to average battery life so its time to get a new battery
Answer:
Only if your battery run out too fast than usually.
Answer:
Whe you unplug it form the wall and it turns off, seriously
Answer:
when it doesn't hold a charge anymore.