This question is virtually impossible to answer because the term “Online Class” is so broad.
Generally you have some kind of material you have to read (textbook, powerpoint, written stuff online, or even podcasts or videos of lectures.)
Then you have to take some form of a test (online, written, av, etc) that you have to submit. Sometimes courses are designed with fairly flexible due dates, sometimes they are pretty strict (ie test 1 must be completed between Nov 22 and Nov 24)
So unless you are asking about a specific school or a specific class, it’s really hard to answer your question with details.