Concept, research, and design = only he could tell you.
I do the occasional comic panel for friends and special occasions. Sometimes these things almost draw themselves. Other times it can take months from the first rough idea till there's a finished piece. Some ideas percolate for years and take several iterations before the panel is what I'd consider "finished."
Actually drawing the strip = my guesstimate is two passes (to block out, blue pencil in, and get the text fitting just right). Figure around 3 or so hours in total. Final 'inking-in' probably took another 1 to 2 hours if he does it by hand and then scans it in. (I don't do enough digital drawing to have any good idea how long it would take doing it on a PC.)