I usually depends on how complex the page is. On a page with a lot of different background, foreground, and character elements, a single panel can take me upwards of an hour, while a full page can take longer than a day.
I usually start by either getting a reference from Deviantart or taking photographs of some action figures for the poses I want my characters in, then I bring them in to Illustrator and trace the lineart.
After that, I'll bring the lineart into Photoshop, mask it and make a couple of color layers: flats, shadows and highlights.
After that I'll go into Google Sketchup and render out the background and foreground elements I need, bring those into Photoshop, composite them in with the character drawings, then add various lighting filters, special effects, text boxes, and any final touches that need to be done.
It's a long process, but it's the best one I've developed so far!