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While three to four days is a good rule of thumb for most leftovers, there are exceptions (for instance, leftover raw shellfish, such as ceviche or sushi, has a lifespan of just one to two days). For an easy way to determine whether to toss something, there are a number of free apps that can help; one is Is My Food Safe?
(a collaboration between the Academy of Nutrition and Dietetics and ConAgra Foods), where you can scroll through by category, say, desserts, and go from cakes to puddings. Another is USDA's new FoodKeeper app
; among other features, it lets you enter the date you made something and then receive notifications when the food is nearing the end of its recommended storage time.