The lobby ambassador can specify the amount of time that the guest user accounts remain active. After the specified time elapses, the guest user accounts expire automatically.
The local user database is limited to a maximum of 2048 entries and is set to a default value of 512 entries (on the Security > General page). This database is shared by local management users (including lobby ambassadors), net users (including guest users), MAC filter entries, and disabled clients. Together these cannot exceed the configured database size.
For the configuration following URL may help you
http://www.cisco.com/en/US/docs/wireless/controller/5.0/configuration/guide/c5users.html