How often should one recite the first or last ten verses of Surah Kahf?

There is no special instruction with regards to it. Prophet Muhammad (ﷺ) only asked to learn, memorize and guard it. Other than that, there is nothing specifically mentioned. However the whole Surah is to be recited every Friday.

It was narrated that Ibn ‘Umar (may Allaah be pleased with him) said:

“The Messenger of Allaah (ﷺ) said: ‘Whoever reads Surat al-Kahf on the day of Jumu’ah, a light will shine for him from beneath his feet to the clouds of the sky, which will shine for him on the Day of Resurrection, and he will be forgiven (his sins) between the two Fridays.’” [1]

However, scholars have considered this last statement, i.e., “he would be forgiven for his sins between the two Fridays”, to be a statement of the companion, Ibn 'Umar, and the attribution of this statement to Prophet Muhammad (ﷺ) is not authentic.

But on the other hand, scholars looking at Ibn Umar says, that such a statement is highly unlikely to be said by Ibn Umar himself, as the information is related to matters of the unseen (ghayb) world, and no one has the authority to make such a statement, without having any proof. So when a companion makes an explanation of this nature, then it is assumed, that it came from Prophet Muhammad (ﷺ).

Reference:

[1] Al-Targheeb wal-Tarheeb, 1/298