Strange. I keep my Apple Pencil in a small case attached to the smart cover - and it drains pretty much from full to nearly dead in a day's time. iPad sits still on my desk most of the day (with pencil unused), Stays in my briefcase a lot of the time when not at work. On night stand plugged in at night. The only "movement" of significance is when I'n walking down the hallway from office to another office or meeting room.
When new - battery would last (under exact same conditions) for about 2 days- maybe three before the low battery notice would just pop up. Now it shows up pretty much each evening.
Apparently Apple now has a battery program for the Pencil - $29...
How interesting. My wife's and my own Apple Pencil can last months on a charge without usage. With usage, I use about 10% per 1-2 hours of writing. My wife uses a bit more but she's also writing a bit more than I am and for many more hours. These pencils are about 6-7 months old.