The process by which yoga is taught to the people that are interested in it is quite similar to how it has been done for thousands of years. Yoga is not something that was invented just a few years ago, quite on the contrary it is the sort of thing that human beings in the subcontinent have been doing since they first decided to drop the hunter gatherer lifestyle in favor of living in agricultural settlements for the most part. Hence, when you make the decision to become a yoga teacher, suffice it to say that you will be taking part in a tradition that has existed for so long that it has become an indelible aspect of the cultural fabric of our species.
That said, the ancient nature of yoga does not necessarily mean that you have to spend countless years trying to master it. Rather, a course like Marianne Wells RYT200 lets you acquire enough knowledge to start teaching it in just 200 hours. If you treat this like a part time course in that you only dedicate 20 hours to it per week, it would take you little more than two months to be able to fully complete it and pass your test with flying colors.
Some might say that 200 hours is not nearly enough time to facilitate a deep enough understanding of the various things that yoga is all about, but these people clearly don’t know what they are talking about. There are plenty of 200 hour courses that can let you teach yoga if you pay enough attention to them, so there is no reason for you to delay getting started with it.