Instructors can set a time limit for a test in the assessment's Timing settings. Once enabled, students see a countdown timer, and their attempt is automatically submitted when time runs out (unless they've already logged out, in which case the "when time expires" setting decides what happens).

To set it up: open the test's settings and go to the Timing section, then set your time limit (and grace period, if desired). If you need help, contact AME Support.