From Xojo Documentation
The number of seconds after a browser disconnects that the session will actually be removed from memory. The default is 180.
This is the period of time that a Session stays around to avoid the overhead of starting up a new Session, should the user come "right back". Use this property to adjust this time up or down based on your system and user requirements. It should not be set too low. Since it defines the amount of time that passes since the last communication from the browser to when the Session is actually closed, setting it too low could disconnect active users.
In most cases you should not need to change this property.
WebSession.UserTimedOut event handler