Too much subjectivity here, let me attempt an anchoring in the phenomenological reality
I think speed factors can be divided roughly into "internal", "external" and "professional".
1. "Internal factors" are those to do with inspiration, imagination, skill, typing speed, focus, discipline, need or lack of such for total immersion for hours or days, need to develop personal techniques or their presence already.
2. "External factors" are those to do with health, job obligations, family obligations, place to write.
3. "Professional factors" are those to do with where one is in the journey--unpublished, published, unsuccessful, successful, franchise writer, original content creator, serial vs standalone novels, genre, complexity, etc.
The realistic assessment of the combination of those three factors at any given point give the basic answer to "how fast will I write this book?"
#
A pro writing for a franchise, living as a bachelor as or a family member in a situation in which the needed time and space is not an issue, and health is not acting up, can knock out an installment in a fortnight or less. Someone with frail health, many responsibilities, and only limited time and space, may take two years or more for something similar.
And even just "internal factors" like temperament and immersion needs can vary greatly, as can be seen by comparing authors who belonged/belong to basically the same generation and wrote/write basically the same things in terms of genre, complexity, length, and success.
Iain Banks could write in a couple of months a novel that would take Martin Amis* two or more years.
John Saul can write in two months a novel which Dean Koontz would need a full year for, while Thomas Harris** would need years and years of agony for that same novel.
Stephen King's The Stand was written in like a quarter of the time Robert McCammon needed for basically an identical novel***, called "
Swan Song".
During the heyday of New Wave fantasy and sci-fi, Roger Zelazny would basically need a month to write what Michael Moorcock**** wrote
in a day. And so on.
#
If, when examining where the Internal Factors, the External Factors, and the Professional Factors intersect for one here and now, one sees that in principle (as in "if only this and this happened") one can write the book in a couple of months, but in practice (because "this and this will not happen") will probably need a couple of years, then the real writing speed, here and now, of this author, is not the imaginary one that needs ideal conditions, but the actual one, that needs to take into account the non-ideal conditions.
Know thyself inside out, know your situation without denial, and the answer on writing speed shall be forthcoming.
____
* Yet in terms of quality and genre it was a "Blur vs Oasis" situation.
** Pearl Jam vs Sound Garden vs Alice in Chains, or, for even older farts, Slayer vs Megadeth vs Metallica.
***The Stand vs Swan Song is a bit of a "Sgt Pepper vs Pet Sounds" situation, but much, much closer than those two.
**** Led Zeppelin vs Black Sabbath.