My experience adding type annotations to a 2.5k-line Python library 
The wwvb package for Python has been a focus of my recent hobby-time programming. I've used it as a place to educate myself about the ins and outs of maintaining a Python package. In the past, I used it to learn about using pylint, black & code coverage to improve the quality of Python code. Most recently, I added type annotations through the whole package until mypy --strict was happy with the whole wwvb package and uwwvb module. The annotations were added in two steps: See pull requests #7 and #8. Together, these PRs contained 320 insertions and 223 deletions across 14 python files, plus 6 insertions in 2 other files related to CI. I did the work during a part of a day, probably under 4 hours of time spent. Since the package currently contains exactly 2500 physical lines of Python code, adding type annotations touched or added over 10% of physical lines!