Should one persist in supplicating against the oppressor, or is it sufficient to supplicate against him once to achieve victory and claim one's right?

Firstly, pardoning the oppressor and refraining from supplicating against him is preferable. However, if you decide to supplicate against him, repeating the supplication is more likely to result in its acceptance. The answer to the supplication of the oppressed against the oppressor might be delayed due to a wisdom known to Allah, similar to the supplication of Moses and Aaron against Pharaoh, which was answered but its fulfillment was delayed for forty years.