Go to System → Settings → Checkout
This value controls the time interval in days before incomplete orders will be deleted. An empty value means that no orders will be deleted.
An incomplete order is an order not confirmed by the customer when he fills all data, selects delivery, and payment method but never completes the order. Please note order cleanup runs in the next new order created.
The example case is when the customer starts checkout (fast checkout enabled) with an invalid address and your current delivery methods do not allow customer to choose any.