If the student has not interacted with the assignment after 25 minutes, a warning message appears stating that the student will be logged off in 60 seconds due to inactivity. The student must click the Keep Me Signed In button to continue. If the student does not click the Keep Me Signed In button within the time frame as it counts down, the student is logged off and sent back to the school's Login page.
If the assignment is a quiz or test and the student stops interacting with it, not only does the inactivity timer appear, but the quiz or test gets locked by the system. However, if the student stops interacting with a quiz or test and opens another assignment before turning in the quiz or test, the quiz or test gets locked by the system after two minutes. When the student attempts the quiz or test again, in any situation, the student can request an unlock by the teacher. Quizzes and tests are not like lessons and projects where students can save their work and come back later. Typically, quizzes and tests may have one attempt to complete the questions. If not completed and turned in, a system lock occurs.
Found on the Student Work-Flow page.