It's a little difficult to give an estimate, as the amount of time spent on a page greatly depends on what's involved in that page in regards to various things, like the view/angel the characters are drawn in, number of characters, number of hands, number of panels, background complexity, amount of text bubbles, etcetera.
I guess maybe around 5ish hours (if it's going well) for a black and white page. (And that may not even be very accurate as I am a lose track of time when I'm working type)
Much much much longer for a colour one, this length only increasing if the background is detailed.