I'm not sure when I had the idea that I could become a yoga teacher but I do remember it was only a matter of two or three days between beginning to research how to become an instructor and applying to a program that appealed to me. The morning after I applied I was accepted, and I was in my first class just 6 days later.
So far my understanding is that there are three levels of yoga trainers recognized in Canada. There are the teachers who have spent 40, 200, or 500 hours in approved studies (and often much more than that in other learning settings such as retreats). I decided to go for the 200 hour certification because I didn't think I could learn all I wanted to, enough to teach the way I'd like to teach, with anything less.
We have three text books as well as videos and of course live demonstrations and our own practice to learn from. It's a serious commitment before even before taking into account that 20 hours of classes are held over three days every two weeks, a 2.5 hour drive from my home.