By default, the autosave feature is run every 5 minutes. This can be changed in the ACF Notes.ini file, located in the program folder.
Open the ACF Notes.ini file and locate the line containing;
Change the value to however many minutes you would like it to be run (minimum is 1), then save the file.
Note: you will need to re-start ACF Notes before your changes take effect.
Note: If you are using the latest version of ACF Notes, you can now change the interval via the Tools > Autosave menu