Light and crispy tempura is best eaten fresh. It loses its delicious crisp texture very quickly when it starts to cool and when chilled in the fridge. That said, it's safe to store leftover chicken tempura in the fridge for 2-3 days. The best way to revive it is using an air fryer or in the oven, but it will lose most of its crispy texture, so it's best to only cook what you plan to eat.