If a person is affected by strong waswas of kufr, then he seeks refuge with Allah, but what's if the thoughts of kufr doesn't dissappear. in this case is it sufficient for him to seek refuge with Allah once or does he have to seek refuge with Allah for every thought of kufr? Jazak Allahu khayran.
All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
When one has such whispers, he should seek the refuge of Allaah The Almighty from them and he should be keen on doing so in order to be protected by Allaah The Almighty from the plot and evil of the devil. However, seeking Allaah’s refuge is not an obligation, so if one does not seek Allaah’s refuge, he is not sinful [this is permissible].
Moreover, one should know that as long as one repels such whispers, they will not harm him provided that he does not commit any prohibition either in words or action. Repelling such whispers is a sign of goodness in the person. Abu Hurayrah narrated that some Companions of the Prophet came to him and asked him, ''We find in our hearts what we consider as a very serious matter and a great sin to talk about." The Prophet said: "Do you really feel this?" They replied, "Yes." He remarked: "That is the essence of correct belief." [Muslim]
The scholars stated that what he meant by ‘that is the essence of correct belief’ is that hating this is the essence of belief, and not the existnece of these whispers. Besides, Ibn ‘Abaas narrated that a man came to the Prophet and said, ''O Prophet of Allaah, one of us finds in himself [satanic whispers about Allaah] and feels so bad that he would rather burn until he became like charcoal than to reveal what he is thinking about.” Thereupon, the Prophet said: ''Allaahu Akbar, Allaahu Akbar, Allaahu Akbar! Al-Hamdu Lillaah (praise be to Allaah) Who limited his plots to whispers only!” [Ahmad and Abu Daawood].
Allah Knows best