For making a demo site it will hardly take 10-15 min for the normal speed of your connections.
You just need to be having your LearnDash already installed and activated.
Now,
Go to the courses page and add new >> Give a tittle >> Some Description and save it
Go to the lessons and add new >> assign it to the course in settings >> save/Publish.
Go to the quizzes and make some quizzes
Go to the topics and make some topics in short for your demo.
Finally, some certificates and you are done with basic creations.
Now it’s time to set the course page over the menus and then visiting the main site and you are done with the demo creations.
If you don’t know how to setup Menus then see here: