like you said, the issue is not Aruba specific, but there are a couple of things you can look at.
The first is the default user idle timeout setting (on the advanced tab of the Authentication section on the controller). This setting is used by the controller to clear idle sessions. When it determines a client is idlel (by the timer) it will send a probe to the client to see if it is online and sitting idle; or truly offline (this includes when an iOS device sleeps). if the client responds, the timer is reset. if the client does not respond, it's session is cleared. In this case, the client would need to reauthenticate the next time they connect (the symptom you are seeing). You can try increasing this timeout; but beware, this is a global setting for all user sessions.
The second relies on ClearPass Guest.....you can get around this by tying MAC-Auth to the captive portal logons. The user may be asked to logon the first time each day; but subsequent attempts can be handled by MAC-Auth of the device.