French and Belgian comic artists produce on average one to two black and white pages a week (add more time for fully colored pages). In Japan, several pages per day is not uncommon. The big difference? Division of labor.
In Asia, a comics page will go through many studio hands: penciler, inker, letterer, "special effects"/toning artist, etc. In Europe, most comic artists work alone and handle every task (except for the coloring).
While mangas have many uncontested strengths, European comics hold the upper hand in favoring authorial voices. In that sense, the creative and artistic processes in European comics resemble more our tradition of children’s books.
American publishers like Marvel or DC Comics factor about three days to complete a page: one day for the penciling, one day for the inking, and one day for everything else (writing, lettering, coloring, etc.).
Presuming an artist wants to create a 100-page children's graphic novel in full color at that pace, it would take about one year of full-time employment (3 days/page X 100 pages = 300 days).
What does this imply? Well, will the publisher's advance sustain the artist for a year? What if the graphic novel is a dud and the publisher could have found out about it with a book half as long? Finally, longer children’s graphic novels don’t bode well for artists who produce their best work on shorter and more varied assignments. At 100-page or more per children's graphic novel, the industry risks weeding out talented sprinters (who could earn more in other trades) in favor of marathon runners and assembly-line worker bees.