When you select "Set a time limit for this test?" in the Custom Test Builder (or when editing a custom test), it will bring up a box for you to enter a time limit in minutes.
Applicants will see a countdown clock next to their test form. Once time is up, the test will automatically submit.
(We track every answer an applicant logs on their test form, so there's no need to worry that the test won't save when it submits.)
For more context, see our complete walk-through of the Custom Test Builder.