I know that Hiro draws pretty quickly (around 30 minutes a page)
Oda must be in that range; perhaps around 40 minutes a page.
Kubo is quite a planner; but still I don't know if he's that fast (around 70 minutes a page let's say).
As for Kishi, I'd say he's in the middle (around 1 hour perhaps). I guess the mangaka increases their speed as they get used to a particular character obviously.
So, let's say that a new manga (with break intervals, refinement details, plot issues, dialogues and post-production) take a total of 4-5 days to be ready.
If the drawing is pretty similar for the next chapter (character-wise) and if he knew his plot well in advance, I guess he makes another chapter in around 2 days more. Which means than in week, he, at times write 2 chapters and at times, one only and take some time off.