I got the following answer from Bookeen (after 6days, ok)
Sir,
Thank you for your interest.
There is currently no possibility to see how many days are left on a borrowed book in the Cybook Opus. But this is an interesting suggestion. We have forwarded your email to our technical team so they can work on this kind of feature.
Best regards,
Bookeen Support Team
Bookeen -
www.bookeen.com