There have been many instances in the past when I had to give myself an ultimatum: I NEED to finish Book Y by X amount of time because of (insert important reason here).
When this happens---
Scenario 1: I create an outline and manage to follow it to the letter by keeping my demonic inner critic on a loose. I worry about the speed in which I've been able to finish the book, but because the story feels "right" to me and I can't think of any other way to write it - ah, f*ck it, que sera, sera.
The book is published, and it becomes one of my bestsellers.
Scenario 2: I create an outline, attempt to follow it but quit halfway and trash the first draft because I keep worrying about what others think. So I go back to square one and write something completely new - something that I'm sure will please EVERYONE (reader, non-readers, critics, fellow writers, I do mean everyone).
The book is published and voilà -
It's a flop. > . <
How much time it takes you to do that is immaterial.
Also, forget about your inner critic. I honestly believe 99% of the time it's not objective, and it's just your insecurities speaking. If you feel you absolutely must listen to it, then do so when you're done writing the book but not while writing it.
As long as you're writing something you love - you're good. :)