I don't know how long the battery life of new one is. However, it is certain that DivX requires a lot of CPU power, and I have read that DivX requires more electrical power than DVD.
So I can't answer your question, however, I have to tell you that
you should test the battery life before the purchase
if you are buying used one. I have been using my iBook for about three years, and it was mostly hooked up to a wall jack. Now, I can't use my iBook without power because the battery life is shortened to about 15 to 20 minutes.
I guess that it happened because the battery is charged and discharged again and again while the computer is hooked up to the wall jack. In case the owner of the iBook used the computer mostly in his/her home without taking the battery out while he/she uses it, the battery life could be shortened a lot. Now I am using my iBook without a battery installed, and I use a battery only when I should.
Please forgive me in case that I wrote some broken English or stupid words.