Battery life time??

Since your question is associated with the Canon A570is, I'm going to assume that you as referring to NiMh batteries in comparison to Lithium-ion. Each battery type has its pros and cons. The Lithium will hold charge better while waiting on the shelf, but it loses total capacity over time no matter what you do. Lithium does NOT need to be run down completely before charging but actually performs best if you top it up frequently. Both types have lots of capacity and will sustain 300-500 recharge cycles before needing replacing. You will want to rotate all your cells through the camera and charger. NiMh ideally should be kept in sets and not scrambled. Get a top quality charger that charges the batteries individually and that will help too. NiMh run down just sitting on the shelf (Lithium does too but much slower. All batteries self discharge its just a matter of how fast). That means, if you are going on a big shoot (vacation) you'll need to top those NiMh up just before leaving on safari because they lose 30% of their charge every month they just sit (Lithium loses 10% per month). If you really want to get all the information about batteries go here:

http://www.batteryuniversity.com/

For the Canon A570is the real advantage is that you can carry along (or buy worldwide) a few AA alkaline as final backups to get your pictures when the NiMh runs out unexpectedly. As you will see if you go to the batteryuniversity site, it is always a compromise of one factor or another. Great shelf life is needed for emergency backup, but higher energy density for your everyday use. NiCd will recharge up to 1500 times but has the dreaded "memory" problem to deal with.
Best Wishes
Larry