Ask for reviews no more often than every 30 days, and no less often than once a quarter.
There is really no science to this answer, but it makes sense. Here's why: A review is a gift. Personally, I am offended when every service I call beats me about the face and neck with invitations to give feedback. I know my feedback is valuable to you. But guess what, my time is valuable too. If you want my feedback about how you can make your service better and more effective, pay me. If not, at least be polite enough to ask once in a while, not every time I call or ask for help.
END RANT
Sorry folks, it needed to be said: stop taking your reviewers for granted. They sense that they are individually rather unimportant. If they were more important, they would be paid consultants not reviewers.
So, don't hound for reviews more than once every 30 days. It is an imposition on your clients.
I'm thinking websites here. If you complete a website for a client and are not in a long-term marketing relationship, you are likely to be speaking to the a lot less soon. Now is a good time to get feedback. How did the project go? What could have gone better? How were their expectations met? Exceeded? Not met?
The rule is the same for any project based business: the end of a completed project is a great time to ask for feedback and reviews.
I have recently begun working with Signpost and have been really happy with their review and referral software. Their software connects to a business phone number and e-mail addresses. Whenever anyone calls or e-mails the business, his or her information is logged into the referral software. Within 30 days the caller / e-mailer is invited to give feedback on the company. The prcoess is good because it is automatic. It is graceful because it is intelligently scheduled. The client is not hit with an invitation for feedback immediately. That seems to me to be a convenience for the service provider not for the reviewer. And the invitation for feedback is sent after service is likely to have happened, and even with enough time elapsing that the client has had some time to live with it.
Here's what I recently sent to my clients. Feel free to copy, paste, edit, delete...:
How is my service? I am starting a new practice where at least once a year I'd like to ask all of my clients for feedback. I appreaciate your business and value your time. If you have any feedback to give me about how your account is being handled, I would really love to hear it.
This feedback is solely for my review and interest in providing the best service possible to you and all of my clients.
Thank you so much for your time.
Short, sweet, to the point. Got any feedback? I'd love to hear it :)
Photo Credit:
Coolcaesar at the English language Wikipedia [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/)], via Wikimedia Commons
https://commons.wikimedia.org/wiki/File%3AFeedbackbumpersticker.jpg