iPad Pencil life expectancy

You're very welcome.

Try warming the Pencil prior to charging (while warm) as suggested. If you're very lucky, Lazarus may return...

If you leave a lithium-ion battery in an uncharged condition for long enough, it will not be able to take a charge. This is just a fact of the way the electro-chemistry works. The problem is more likely to happen in something like a pencil because the battery is so small. It will drain more quickly than an iPad.

If you had been using and regularly charging the Pencil, the battery would also eventually wear out. I'd guess two-three years.

A regularly charged and maintained Pencil can be expected to last for many years. By contrast, a rarely used Pencil that is not regularly charged can be expected to suffer terminal failure within a very short timeframe.

The tiny internal Li-ion battery is susceptible to permanent/irreversible damage through being left discharged for long periods. Even some “new” pencils can exhibit signs of failure out-of-the-box if they are “old stock”. It is essential that if you have an Apple Pencil that you charge it regularly - whether used or not - so as to protect the battery from deep-discharge. Do not allow a pencil to remain in low-charge state for any period of time - as the internal battery will fail, rendering the Pencil useless. Setting aside an unused Pencil, for extended periods, is a recipe for premature death of the Pencil battery. In common with all Li-ion batteries, they must always have some level of charge.

Gentle warming of the Pencil (such as placing on a hot water radiator), prior to attempting charging of the Pencil, can sometimes resurrect a dead Pencil battery.

If the Pencil Battery has failed, the only remedy is to replace the Pencil. If the battery has failed and your pencil is within its one-year warranty, you should look to having it replaced by your retailer or at an Apple Store.