A testimonial sample does two jobs: it shows a customer the size and register of the answer you are hoping for, and it gives you a standard format for what you file afterwards. This page is both, one sample and the fields the finished record needs, so you can publish it later without going back to ask again.
The sample you send
One example, two sentences, with a note saying anything in their own words is perfect. Longer samples produce imitations; shorter ones produce a sentence with no substance. Send it inside the ask itself, not as an attachment: a sample somebody has to open is a step, and every step costs you a share of the replies.
The format you keep
The words, the customer's name, their role, their company, the date, whether it was written or filmed, the permission to publish it and where it is currently shown. Seven fields, and the last two are the ones everybody forgets.
Why the record matters more than the wording
A quote you cannot attribute, date or prove permission for is a quote you will not use. On the worked example 18 arrive a year, and the difference between eighteen assets and eighteen emails is whether anybody wrote them down properly.
Questions people ask about testimonial sample
Should the sample be from a real customer?
Yes, once you have one. Before that, describe the shape instead of inventing a person: a fabricated testimonial is exactly what the FTC's rule prohibits.
How long should I keep them?
As long as you publish them. A testimonial on a live page with no permission on file is a problem waiting for somebody to ask.
Do I need the date?
Yes. A four-year-old quote about a product you have rebuilt twice is a liability, and you cannot spot one without the date.