It all depends on what kind of yoga instructor you want to be.
Do you want to be the kind that has significant wisdom, the kind that will add value to people’s lives? The kind that will challenge or support your students, knowing when to do one versus the other? This takes time.
Do you want to be the kind of teacher that knows anatomy? …not just from a book, but also from personal experience and experiments with your own body’s mechanics, flexibility, and limitations? This takes time. And let’s not forget that as a yoga teacher, you’ll be helping other people with their bodies too, so learning about other people’s body mechanics, flexibility and limitations takes time, as well.
Do you want to be the kind of teacher that knows yoga as a philosophical pursuit, as well as a psychological one? Yoga is a path to study, and ultimately liberate, consciousness. There are many schools that developed, and many more that faded away, over millennia, each offering a variety of perspectives on the biggest questions in Life, and each worth studying, appreciating, adopting, rejecting, and overcoming. This takes time.
Alternatively, you can spend the next eight weekends intensively memorizing a bunch of information to obtain a 200 hour certificate, if that’s the kind of teacher you want to be. This doesn’t take much time at all.
“Yoga is an internal practice. The rest is just a circus.” -Sri K. Pattabhi Jois