You'll end up spending ~2-3 full time months creating each course. Then you need to market it (or use a platform but then you don't build up a customer list of your own that you can use later), and on top of that you need to support all of your students by answering questions (not as bad as you would think but it's something you need to do constantly).
Then in 6+ months when the tech you've created your course on gets updated you have to go back and re-record your course to keep it relevant. There are some tricks to minimize this burden depending on what tech you're creating content for, but I would say as a whole expect to re-record the entire course every year. By this point you'll end up re-planning the entire course, so it's not a simple thing.
This doesn't mean it's a bad idea, but don't get the idea that you just create the course in a few tough weeks and then sit back on a beach while you sell dozens of copies a day for years on end.