Hello Velarixter,
Thanks for posting your question regarding the Surface Pen's battery life in the Surface Community Forum.
We've decided to convert your post into a discussion. There is no indicator for the Pen's battery, and as mileage for the pen's life will depend on use of the pen, it seems appropriate to have this be a discussion open to other community members' anecdotes and experiences. You and other members are welcome to discuss possible improvements for the Pen as well. One last thing: you did not indicate whether you meant the battery life for the AAAA battery or the coin battery in the top half of the pen.
Thanks for posting in the forum, and let us know if you have any other questions or issues.
Best Wishes,
Zack_T