Answer by Robert Frost:
There’s a big range in the time taken to complete a page, depending on the techniques and style used by the artist to tell the story. In the above image, I’ve included pages from Gilbert Hernandez, Jack Kirby, Charlie Adlard, Frank Cho, and Jay Anacleto.
In general, if an artist is assigned to do every issue of a monthly book, and an average comic book is 20 pages of art, then the artist needs to complete a page a day if they want to take weekends off to spend with their family.
However, while in the 1960s, Jack Kirby could churn out three pages in a day, today the audience demands a greater amount of detail and rendering. Printing technology has improved and the audience expects the artist to take full use of that improvement. Many of the most in demand artists cannot meet that monthly schedule. They require either greater lead time or they will have fill-in artists do a few issues of the book each year to allow the artist to have more time for the issues they draw, or they will work seven days a week.
Here are a few tweets of artists talking about their efforts to complete pages.