No. 21 days is just the name of the course, originally a book. You can go at your own pace. I have been doing a topic or two then play around with what i have just learnt to get it into my head.
So no you can take as long as you want and the chapters are quite long so you may want to spread them out.
You might be able to do it in 21 days but don't force yourself through it other wise at the end you will think what have i learnt and you won't have taken in what you wanted as you were rushing so to speak.