Hey I was wrong, it has made me solve my problems !
If I put met session.cookie_lifetime to 60secs, the session cookie will be wrote and its expiration time will be reevaluated on every session_start()
So if my user stay inactive for 2 minutes, session variables won't be set any more.
The only bad point is that the session will keep on living after the window closing. But if the lifetime is small, it won't be a problem
So my problem is solved: it is possible to have a session expiration after an inactivity delay :p