I recently ran a poll on Twitter to ask my fellow #acwri community how long the writing up stage of developing a manuscript takes. Not to my surprise, the majority voted that it widely depends. I, too, have experienced that not all papers are born equally. Sometimes I can knock out a draft in 12 hours. Sometimes, I keep changing the introduction to make sure I get the right approach angle and get the work to stand on its own.
Below you can find the wake of this poll: