For Windows version 1.x, the database is stored at:
C:\Users\Username\AppData\Local\Paprika\Data\
For Windows version 3.x, the database is stored at:
C:\Users\Username\AppData\Local\Paprika Recipe Manager 3\Database\
Note: The AppData folder is hidden by default on Windows, you might need to enable "Show hidden files and folders" in Explorer's options to be able to see it.