What's the average cycle count and lifespan of a PB G4 battery?

If it's a Lithium-Ion/Polymer battery the average lifespan is about 500 full recharge cycles.

First, you should know that Applecare doesn't cover the battery, it is specifically excluded unless it's defective. That's because Apple considers the battery a consumable item. You can read it under limitations at http://images.apple.com/legal/applecare/docs/AppleCareProtect_Plan_NAen.pdf

Regarding the life, how you maintain the battery has an impact on how many cycles you get out of it. For example, if it's plugged in all the time, your cycle count will be low by the time the battery fails. Apple says that a properly maintained battery should be at 80% capacity after 300 cycles. This is the Apple note on battery maintenance: http://www.apple.com/batteries/notebooks.html

An "average" number isn't very useful for this subject. Your battery life can be lengthened or shortened depending on how the battery was treated, with respect to your charging habits/typical charge level and its typical temperature. See the table near the top of this web page.

Read my previous reply.