When to Approach an Editor
You have finished your book. You have revised it, corrected it and read it more times than you care to remember. As far as you are concerned, it is ready to publish.
This is the time to approach an editor.
Do Not Leave It Until the Last Minute
Editing takes time. So does dealing with the edit afterwards.
A copy editor may return a manuscript containing corrections, suggestions and queries. Some will take seconds to deal with. Others may require you to check an earlier chapter, reconsider a passage or decide between two conflicting pieces of information.
Allow time for this before you set a publication date.
Contact an Editor Before You Need One
Editors book work in advance. An editor being available today does not mean they can start an 80,000-word novel tomorrow.
An editor will require some basic information about your book before taking on the work. They may want to know the genre, the timeline, the setting, the period in which it is set, the word count and whether you have a deadline for publication. They may also request a sample of the work to determine how much editing is required.
It is always advisable to approach an editor before you have made your final pass. A good editor will want to know something about you and your book before deciding whether to take on the work.
Leave Room for the Unexpected
You may expect the edit to uncover a few spelling mistakes and misplaced commas. Most of the time it won't be a simple spellcheck.
An editor may uncover conflicting dates, a character whose description changes, an event that happens in the wrong order or information that appears in one chapter and is contradicted in another.
These require clarification. An editor can't decide which one is correct and you will have to make yourself available to answer these sorts of questions.
The right time to approach an editor is when you think your novel is complete and you still have time to make any changes, answer any questions and rework before a deadline.