Expected lifespan of Magic Wireless Keyboard

AFAIK, Apple hasn't published a specification on this keyboard that includes the expected battery life.

This keyboard uses a rechargeable lithium battery. One thing about these batteries, not just in the Apple keyboards is that, if not used for an extended period of time with the battery near or fully depleted, it may no longer accept a charge. By extended, I would mean a number of months or years.

Besides battery life, you may want to closely examine the keyboard's charging port to make sure no type of debris worked its way inside.