Take it your looking at timing out the session if the client connects to the WLAN but doesnt finish the captive portal? Rather than trying to display a timeout on a webpage as a count down?
Check out this https://www.cisco.com/c/en/us/support/docs/wireless/wireless-lan-controller-software/118963-config-web-auth-timeout-wlc-00.html:
config wlan security web-auth timeout 3600 <Wlan_id>
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