This is a very interesting point to bring up. As others have mentioned, revalidation is necessary when the product or device has been modified or updated from the original design. However, if there are no changes and the device is functional, then there is no need for revalidation of the product. However, the original validation would make sense to expire at some point. There is no exact duration that the original validation can be useable. The expiration of the original validation would be determined appropriately according to the product itself. In addition, another reason a revalidation would need to occur is if there is a change in the validation process itself. If there is a step added to the validation process, it is important that the products are revalidated. If not, then they will not follow protocol and become outdated. The validation process is a very important developmental stage for products or devices and should be maintained.