Automatic saving
The game automatically saves after each map change. When planning to stop, use a completed map transition as the known saving trigger rather than relying only on how long you have been playing.
Before quitting
Do not assume that opening a menu saves the game, or that every activity writes progress immediately. The map-change rule does not identify a particular save icon or an exact safe-to-exit delay.
If progress loss is your concern, keep that distinction in mind before quitting straight after an action. This page does not promise that every exit timing is loss-free.
Back up a Windows save
You can copy Save.lunne separately before troubleshooting or making edits. See the Windows save location and backup steps.
Backing up an existing file and triggering an in-game save are different tasks: copying the file preserves whatever progress it already contains.