Add the leftover french fries into the air fryer basket. Spray or brush with oil if needed (skip this step if the french fries were originally deep-fried).
How long to cook them depends on how many french fries are in the basket and how thick they are. For thin french fries or about 1 cup of french fries check them in 3 minutes. For thicker french fries or 2 or more cups of french fries check at 3 minutes and shake them around. Continue to air fryer them checking every 1-2 minutes.
Serve the crispy french fries with your favorite dipping sauce.