Momentum answers
When is the best time to ask a client for a review?
As close to job completion as possible — same day if you can manage it. Satisfaction is highest immediately after the work is finished and fades within days, so a request sent a week later typically gets a noticeably weaker response than one sent within a few hours.
This is also why the ask needs to be tied to an actual completion event rather than a fixed calendar reminder — a job that runs long or finishes early throws off a fixed schedule, but a trigger tied to 'marked complete' stays accurate.