In order to provide fair access to all, while allowing students to get their work done, session limits are enforced. Decisions are made by IT staff in each school to set limits regarding the maximum time you may have an active session, and also the amount of time you can be disconnected from a session, and return to it to complete your work.
When you are logged in and working your session is in an Active state. When you leave your session without logging out, your session goes into a Disconnected state. You should always logout when you have completed your work
If you feel that session limits are impacting your abilty to complete assignments, you should contact the appropriate team responsible for VLab offerings in your school. VLab Support Contact List.