If the concept, audience or chapter order is still changing, a consultation or outline review may be more useful than a full copyedit. If the manuscript is complete, an editor can assess the whole reader experience and recommend the right level of revision before detailed corrections begin.
Ask what the proposed service includes, what files the editor needs and how feedback will be delivered. A transparent scope protects both sides from mismatched expectations. The right time is when you can clearly explain the question you need help answering.