BE battery will last about 2 years under normal usage. After your battery is dead, you can either buy one from Casio for around $60, buy one from another company for around $40 or buy a similar type of battery even cheaper. However, the last option involves some tweaking and soldering.
I've heard that even a dead battery can be revived by running it down completely by playing an mp3; then charging it fully without using it at the same time; and repeating this cycle a few times.
I have also heard that battery life can be extended by avoiding to let it run down completely. In other words, it will serve longer if you connect it to AC cord as often as possible.