First things first: you have total control over when exams happen.
Start by going to the Overview tab in the relevant classroom.
Scroll down to where it says "Class Schedule." It'll look like this:
You can change the due date for the exam, or you can check "no due date" if you want to leave it open-ended.
If the test/exam is only available on a specific date (because you chose a date in the box), then the student will only have access to take the test in the 24-hour window of the test date.
If the test/exam is open (because you chose "no due date"), then the student can access it at any time.
Clicking into a test will take the student to a reminder of the test guidelines and honor code and provide a link to open the actual test. Once this link is clicked, the test clock will start ticking and the student will have 60 minutes to complete the test. Please note: the clock keeps ticking even if a student closes the test and walks away. For that reason, you should remind your students that they should only click “Take the Test” when they are available for the 60-minute block to complete it.
If you want to allow test retakes for the whole class, you can do so via the "Class Settings” area on the Manage tab of your classroom:
Remember: retakes override the previous takes, so any previous work/scores will be overridden if the student clicks in to retake a test or activity.