In the Advanced settings under the WLAN configuration you can configure a "grace period"
This is the time a client can be disconnected but stay authenticated in the ZD user list
The max value is 144000 minutes - 24000 hours - 100 days.
This is different then idle timeout - idle timeout is the time after a client is no longer seen by an AP after which it clears client data for that device. If this is set too long AP's will get overloaded with client's and may not let new client connect. Even if a client device is not actively sending user data the wirelesss card is communicating to the AP and stays associated to the AP. If the client turns off the device or moves away - idle timeout will clear out the AP freeing up space for more devices.