Isn’t it here in the documentation?
" Every server found to be idle for more than 10 minutes will be culled."
Also, see this discussion in this Jupyter discourse forum.
Those are the first and second hits Google returns if you search ‘tljh inactivity’.
TLJH works with the free mirco tier on Amazon which is vastly inferior to what you’ve provisioned. Your server is definitely not undersized at this early point in your tests.