How long do students have access to my course?
Students can access your course indefinitely!
They'll always be able to go back and review older lessons & activities, even if the course has officially ended.
If you want to limit access to your course, you can remove people via the Students tab in the course editor.
Setting a finish date/milestone
Another approach is simply to set a milestone for the end of your course.
Say you want your course to end on June 1st.
You would emphasize that date throughout the course, and then on June 1, you'd send an email saying "the course is officially over," thanking everyone, and providing next steps (e.g. signing up for your follow-up course).
At that point, you no longer respond to comments/questions in the course, but participants are free to go back and review the content if they wish, which can be really helpful for reinforcing their learning over time.