The Quran

استَغفِر لَهُم أَو لا تَستَغفِر لَهُم إِن تَستَغفِر لَهُم سَبعينَ مَرَّةً فَلَن يَغفِرَ اللَّهُ لَهُم ۚ ذٰلِكَ بِأَنَّهُم كَفَروا بِاللَّهِ وَرَسولِهِ ۗ وَاللَّهُ لا يَهدِي القَومَ الفاسِقينَ

Muhammad Asad

[And] whether thou dost pray [unto God] that they be forgiven or dost not pray for them - [it will all be the same: for even] if thou wert to pray seventy times¹ that they be forgiven, God will not forgive them, seeing that they are bent on denying God and His Apostle. And God does not bestow His guidance upon such iniquitous folk.²

I.e.. many times. In Arabic usage, the number "seventy" often stands for "many". just as "seven" is a synonym for "several" (see Lisan al-'Arah and Taj al-'Arus). It is evident from many authentic Traditions (recorded, among others, by Bukhari and Muslim) that the Prophet often prayed to God that He pardon his enemies.
I.e., "those who are so deeply rooted in their iniquity and in their insolent persistence in evildoing (tamarrud) ... [that] they have lost all disposition for repentance and belief" (Manar X, 657).