Sessions are created when the user logs in to the hotspot. The length and validity of a sessions is totally dependent on the operator's settings. First the validity of access can be set to 1 hour, 1 week, 1 month, etc, or set a traffic limit to it. If the access expires, the session is closed too.
If the user is away (disconnects from the wifi), it is up to the router's settings how long the router is keeping the connection for the same session. This has to be set in the control center (idle timeout) and in the router (dhcp lease). If the users comes back within that time, the same session is continued. User doesn't have to log in again, he/she can use the internet immediately, without having to go through the splash page again. If the user is outside those limits, a new session will be started.