Hello Jason,
Yes, Edit your WLAN, and expand the Advanced Options section. There is an Inactivity Timeout (Idle timer) for how many minutes a user stays connected if not doing anything, and also there's the Grace Period which allows them to re-connect before X amount of time without having to re-authenticate. Otherwise, you can use the Service Schedule and only service your Guest access between certain hours of the day, per week.
But I don't think we can totally limit a Guest user to a 20 minute period of use, then kick them off (unless something from a RADIUS server can do this per User/Group permission/AV pair).
From KBA-2433: Maximum values for inactivity timeout and grace period
Inactivity timeout and Grace period can be set on WLAN at ZD web UI >> Configure >> WLANs >>(Edit a WLAN) >> Advanced options
Maximum values :
Inactivity time : Enter a value in minutes after which idle stations will be disconnected (1 to 500 minutes).
Grace period : Allow disconnected users a grace period after disconnection, during which clients will not need to re-authenticate. Enter a number in minutes, between 1 and 144,000.
