Instant Payment Notifications (IPNs) are sent back to the NotificationURL in (but not limited to) the following scenarios their:
When a payment on the PayU redirect either fails or is successful (3D secure redirect)
When transaction pending review for fraud is either approved or rejected by case managers. In the case of approval the IPN will fire after the payment has been attempted and will return that result.
For a detailed overview of the process please go to the Instant Payment Notification section here