The links to purchased products are good indefinitely--that is our intention.
That said, we strongly recommend that you immediately download and back up your purchases. Several years ago, when we changed websites, the customer-stored links would not transfer to the new site, so customers no longer had access to previous purchases. (If this has happened to you, please let us know.)
You can put a back-up copy onto your own storage media--for example, a CD, external hard drive or thumb drive. As long as the copies are for your own family's use, there is no copyright infringement.