There is no single answer, because the employer chooses which tests make up your assessment. What we can tell you is exactly where to look.
Every test shows its own limit before you start
Each test has an instructions page. It tells you what the test contains and how long you have, and nothing starts until you choose to begin.
So you can open your assessment, look at what is in it, and add up the time before committing to anything. Looking costs you nothing.
Some tests have no clock
Personality and motivation questionnaires are not timed. There is no clock on the screen and no deadline to race. They still take a little while, because they have a lot of short questions, but you can take them at your own pace.
You do not have to do it all in one sitting
You take your tests one at a time, and you choose the order. You can finish one test, close the window and come back to the next one later using the link in your invitation email.
What you cannot do is pause a test once it has started. The clock on a timed test keeps running even if you close the window. So the right unit to plan around is a single test, not the whole assessment.
See taking your tests one at a time.
Is there a deadline for the whole thing?
The employer can set one. If they have, it will be in your invitation email. A deadline closes the assessment for everyone, so the time limit on an individual test and the employer's deadline are two different things.
If a deadline has passed and you still want to take part, contact the company that invited you. Whether to reopen it is their decision rather than ours.