It's a little hard to estimate because lately my process has been broken down a lot; I never finish a page from sketch to scan in a single sitting anymore. I usually sketch out a batch of 3-7 pages, ink them gradually over the next couple of days, and then tone them all at once when I have a free evening.
Probably all together, the process takes around 3-4 hours per page? Some are faster, while others (especially pages with complex backgrounds or lots of panels) might take longer. I don't think I've ever spent longer than 5 or 6 hours on a single page, though (not counting covers or related illustrations).
For instance, this page was probably 2 hours or less altogether:
While I completely lost track of the time for this one and went a little overboard: