For Demon House (in color, moderate detail): about 4 hours for one page, but I usually do 2 pages per update for this series so 8 or more in total.
For Heavy Horns (b&w with gray tones, most detailed of my styles): about 4-5 hours
For Erie Waters (b&w only, most simplistic of my styles): about 2-3 hours
I always start by laying the text down with derpy looking sketches to get the composition of the panels looking good, I suppose this is the thumbnail part for me. Then I do a more refined sketch over that which is the base for my line work. Keyboard short cuts and the lasso tool are my friends (even if the latter can be frustratingly finicky at times).
Definitely try and sketch out the page ahead of time because for me usually the layout and composition part of it takes the most time, like I can't think of a good comp or flow for the panels, etc. And when I'm stuck I tend to surf the web and get distracted. lol.