Not giving a time estimate, unless there is a reason for a deadline. Instead asking artists how long it would take them to complete something.
You may also want to adjust your phrasing, because typically in webcomics a "chapter" is at least 20-30 pages. It takes the place of "issue" in print comics for most people. Saying you estimate it to be 40 pages is far better information as the number of chapters won't matter to an artist judging whether or not they can do the work.