An assessment is not one long sitting. It is a set of separate tests, and you take them one at a time.
Choosing where to start
Your assessment page lists every test the employer has included. Each one shows what it is and, if it is timed, how long you have.
You choose the order. There is no advantage to any particular sequence, so start with whichever you feel readiest for.
Nothing starts until you begin it
Opening your assessment, reading the list and looking at a test's instructions page all cost you nothing. A test only begins when you choose to begin it, and the clock starts at that moment.
This means you can look at the whole thing first and plan your time before committing to anything.
Stopping between tests
When you finish a test you go back to the list, and the next one is waiting. You can carry straight on, or stop and come back later using the link in your invitation email.
Stopping between tests is fine. Stopping during one is not. There is no pause, and a timed test's clock keeps running once started, so the natural place to take a break is between tests rather than inside one. See how your answers are saved.
If the employer set a deadline
Some assessments have a closing date, which will be in your invitation email. That is separate from any individual test's time limit. As long as you finish before the deadline, spreading the tests over a few sittings is perfectly normal and the employer does not see how you split them up.
Knowing when you are done
Once every test is complete you reach a completion screen confirming that all tests were completed and your answers submitted. Until you see that, you still have something outstanding. See finishing a test, and finishing your assessment.