Hey Michal,
A customer can certainly image the same machine and and use the same image on the same machine. However they won't be activated if they try to move that image to another machine (because the fingerprints will be different).
Also, in this case, where a customer makes an image and restored it at a later date (like in your example) they'll only be able to use your app for a maximum of nDaysBetweenChecks if you happen to revoke their key. But that would be the same as if they never bother to clone their drive.
If you want to check more often I would recommend 14 or 7 days. Anything lower than that is overkill.