When I start a new chapter for Time Gate, I start the sketches and panel layouts and text and finish all of the preliminary stage in about a week, if the work time is good and if I'm not bogged down by school/Starbucks work/etc. I do these a week or so before the chapter begins in the next month.
As for the linearts and tones, I'm usually able to finish a simple page in about a couple hours. There are times I've spent up to twelve hours in a day just drawing pages, and able to get four or five pages done if it goes well. Bigger and more intricate pages take more time though, of course.
I'm actually considering doing weekly chapters instead of monthly chapters when I'm finished school, as I'm getting about 20-30 hours a week at the 'bucks right now, and I'll have more time to dedicate to comics. But it depends on how the process goes with it.