I'd like to know from you how long it takes you to write a single page.
Once, I made a special effort to ask all the prolific publishers in my university how long they take to write a page.
Without hearing each other's answers, they all said - About an hour - provided I have all the material and have written something similar before.
An hour per page - for experienced writers, for confident writers. On subjects they know well and have written about before. On material which has no gaps.