The user idle timeout will clear the user from the user table if they are inactive for 3600 seconds by default. This is configurable under Security > Authentication > Advanced.
6.3 User Guide explanation:
Maximum period after which a client is considered idle if there is no wireless traffic from the client.The timeout period is reset if there is wireless traffic. If there is no wireless traffic in the timeout period, the client is aged out. Once the timeout period has expired, the user is removed. If the keyword seconds is not specified, the value defaults to minutes at the command line.
Range: 1 to 255 minutes (30 to 15300seconds)
Default: 5 minutes (300 seconds)