As long as it has been refrigerated in the refrigerator, the majority of store-bought oat milk needs to be consumed within 10 days of opening. Usually, the carton will have a place where the "best by" date is noted. Because homemade oat milk doesn't have the preservatives that are typically added to products produced with oat milk from stores, it only keeps for five days in the refrigerator. To determine when to consume your oat milk, make sure to identify the date that it was made.