There’s a loaded question. Some people would answer this based on professional timelines and production schedules. A talented penciler or inker (sometimes the same person is both) should be able, at the professional level to do about a page per day. Remembering that concepts, layouts, panel structure, angles, zoom, and following script direction all take painstakingly detailed attention and time to do so.
Another way to answer this question is to approach it from an inclusive ideology that the time it takes to finish a page is from the time the script page is started, throughout the editing phases, page concepts, pencils, inks, colours, lighting, lettering, page prep, digital layouts and formatting, clearing, and arguably, the printing itself as a matter of page production artistry.
The short answer is a single page in a graphic novel or comic takes hours of work by any number of people, depending on the project. It is with this in mind that I have included a time lapse video of a small portion of that work cycle.
The digital inking (above) and colouring (below). Inking and colouring was done by Allison Danger on pencils created for Blood and Motor Oil by Christopher Ables.