Are you using the controller internal captive portal ?
Check the following :
(CONTROLLER) #show aaa timers
Global User idle timeout = 300 seconds
Auth Server dead time = 10 minutes
Logon user lifetime = 5 minutes
User Interim stats frequency = 600 seconds
(CONTROLLER) #show wlan ssid-profile GUESS-SSID
-------------------------------
Parameter Value
--------- -----
SSID enable Enabled
ESSID guest_ssid
Encryption wpa2-aes
Enable Management Frame Protection Disabled
Require Management Frame Protection Disabled
DTIM Interval 1 beacon periods
802.11a Basic Rates 6 12 24
802.11a Transmit Rates 6 9 12 18 24 36 48 54
802.11g Basic Rates 1 2
802.11g Transmit Rates 1 2 5 6 9 11 12 18 24 36 48 54
Station Ageout Time 1000 sec