Depending on the length of the book and the amount (light, heavy, etc.) and types of editing needed (content, copyediting, proof, etc.), a book can take up to six months to edit (see YouTube video on the editing process and the different types of editing: https://youtu.be/3vWEWoIXrlM). But for the average book, it takes around three months. There are a few things to factor in, though, such as the type of editing needed. For example, a Path To Publishing content editor will go through the manuscript a minimum of two times. After each round, the manuscript will be returned to the author to address issues/suggestions made by the editor. So, we have to take into account the length of time the author may keep the manuscript in their possession.
A content editor will spend anywhere from three-four weeks doing the first round of edits. The editor will then return the manuscript to the author, who will spend, at minimum, two-three weeks with the manuscript addressing the editor's notes. The author will then return the manuscript to the content editor who may spend another two-three weeks doing a second round of edits. The author will then address the second round of notes (which will take between one-two weeks), and then return to the editor. The Path To Publishing content editor will not go through the entire manuscript a third time (unless heavy editing was required). He/She will, though, edit all new tracked content the author may have added or deleted. If the editor has no questions regarding any new content, the manuscript is then passed on to the copyeditor, and so on.
For an editing quote and free sample edit, email your first three chapters to firstname.lastname@example.org.