We think Impossible™ products taste best when served hot, immediately after being cooked -- digging in is the best part, after all. That said, you can feel comfortable storing and reheating it in the way you would other leftovers, as our products pose no unique food safety concerns.
Keep in mind that the length of time any leftovers (not just our products) are safe to eat is largely dependent on how they were handled before, during, and after cooking, so we encourage you to use your best judgement based on the way the leftovers look and smell.