What is the life expectancy of the battery in the Apple Pencil? Can it be replaced or would I need to purchase another stylus?

As FoxFifth is sort of dancing around, when the lithium battery dies, you will have to purchase a new Apple Pencil.

Most Bluetooth Smart Stylii have lithium ion batteries that are not user serviceable without ruining and or destroying the device.

Depending on the size of the rechargeable Lithium Ion power cell in the Apple Pencil and how often it is charged/discharged, it should last, at least, a few years or so.

The rechargeable lithium ion power pack in the Apple Pencil is a custom design and probably not replaceable by user or Apple, at all.

According to iFixit (from https://www.ifixit.com/Teardown/Apple+Pencil+Teardown/52955 )

• The pencil is clearly not meant to be opened or repaired; you can't get inside without destroying the device.
• The layers of plastic and metal holding interior components are impossible to remove without shredding.
• The battery, enough for a 12 hour run, is impossible to replace, giving the device a limited lifespan.