After some testing I found out that a value of "0" does not allow to enter different username/password. After discussing with Henrik Brinkmann from FMS we came to the point to set it to "-1".
The full story/ use case is:
We want to give all users 30 mins free access per day after the registration. Users should be able to login with up to 2 devices.
Additionally we offer paid access during the recharge process to extend the access. There is also the option for the manager to create cards for the login without payment.
Now I ran into the problem that I see people registering again and again for the 30 mins free with different username but same MAC address.
Using a value >0 for the "Login Expiration for Mac Address" does not solve my problem. Some people maybe register for the free product and get later a card with different username. If the value is set to high they can't switch the username.