It's kind of tricky to measure, as I do my work in stages - and it definitely varies from page to page.
Layouts/storyboarding is my biggest obstacle, so that one usually takes most time - I take entire days off to thumbnail, script and plan an entire chapter in one go, and if I'm lucky, I manage to plan the whole chapter (which varies in length between 18-35 pages, depending on content; I don't have a set length). If I'm not, I might need to take another half-day.
Then, I open Manga Studio and do digital roughs - placing the panels and speech-bubbles, doing stick-figure style sketches to give myself an idea of what goes where, working off of my thumbnails. This can take up to a day as well, depending on length (or less, if I've got a good workflow going). After that, I do clean sketches, which takes 30-45 minutes per page (I'm a messy sketcher). After that, I ink a bunch of pages, and then go back and colour the first page - and then skip back to where I was inking and ink one page, and then I go back, etc., etc. This way, I switch between colouring and inking so that I don't get bored and lose attention.
But yeah. The finishing of each page roughly breaks down as follows (low end for simpler pages with fewer panels, upper end for complex pages):
Clean sketch - 30-45 minutes
Inks - 45 minutes-2 hours
Colours - 3-6 hours
So in total, finishing a page takes me about 5 hours. Sometimes it's a lot quicker, but as I said, it depends on what's on the page. The chapter I'm working on right now has a LOT of lighting-effects and detail, so it's a lot more time-consuming - but I have found a nice, efficient way of coloring, and I can tell I'm speeding up the more I draw.