Offering your customers a full refund if they aren’t satisfied with your products is an indication of quality. Or rather, your confidence in the quality of your product. Money-back is the most common guarantee and has almost become standard practice in retail.
Nevertheless, while offering it may reduce perceived risks and help you distinguish your products amongst competitors, there will always be customers that try to take advantage. For that reason, money-back guarantees are usually constrained by a time period, usually 30, 60, or 90 days. Sometimes they can last up to a year or even an entire lifetime.
To determine how long the guarantee should be, you need to examine the specifics of each product. Try to establish how long it takes for your customers to use or wear out the product, and add on a little more time. The longer your guarantee, the greater the risk of customers cashing it in, but also the more sales it will drive.