When a user is idle without any communication with the AP for the amount of time set as User Idle Timeout, the client is deauthenticated by the WLC. The client has to reauthenticate and reassociate to the WLC.
How to change idle timeout:
User Idle Timeout on the WLC GUI , go to the Controller menu. Choose General from the left-hand side to find the idle and User Idle Timeout fields.
when it gets disconnected, i believe it gets disconnected from the wifi, when you say re-logging in, do you mean you have trouble logging in as a webauth guest user, or you cannot connect to the wireless part at all?
you can debug the client and check what happens when the client is unable to connect back.