the only default timeout is the idle timout of 30minutes. That is if the client doesn't rx/tx data for 30minutes the controller deletes the client session. That normaly only happens if the client leaves the coverage area for min.30 minutes.
You'd find the timeout in > VNS > WLAN Services > WLAN Service XYZ > Advanced > It's the post idle timer in the below screenshot...
If the controller deregister a client you'd find the reason in > Logs > Station Events > put the MAC of the device in the field in the upper right to filter.
Here a example of my client after I've left the coverage area... 08/27/15 09:18:16De-registration9C:FC:01:1C:01:D6---D8:84:66:02:DF:E0--VNS[SecureAccess] Cause[Idle timeout]