how much a person should sleep acc. to 51.17,

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, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

There is no specific number of hours which a person is obliged to sleep in the sense that if he sleeps less hours, he is sinful.

What is Islamically required is that a person should give his body its right of rest and not overburden it with something it cannot bear as per the Hadeeth narrated by 'Amr Ibn Al-‘Aas that the Prophet said to him: "Have I not been informed that you pray all night and fast all day?" I (i.e. 'Amr Ibn Al-'Aas) replied, "Yes, O Prophet of Allaah!" Thereupon, the Prophet told him: "Don't do that; pray and sleep, fast and break the fast, as your body has a right over you, your eye has a right over you and your wife has a right over you…” [Al-Bukhari and Muslim]

As regards the saying of Allaah (which means): {They used to sleep but little of the night.}[Quran 51:17]; then Ibn Katheer said regarding its interpretation: “The interpreters of the Quran differed in opinion about its meaning and held two views on it, one of them is: Seldom did it so happen that they spent the whole night sleeping and did not spend a part of it awake in the worship of Allaah. Ibn ‘Abbaas said: “No night would pass except that they would pray even for a small part of it.” Besides, Qataadah said quoting Mutarrif Ibn ‘Abdullaah: “Seldom did it so happen that they spent the whole night sleeping and did not spend a part of it awake in the worship of Allaah either in the beginning of the night or in the middle of it.” Moreover, Mujaahid said “Seldom did it so happen that they spent the whole night sleeping without praying (the night prayer).” Qataadah said the same thing. Furthermore, Anas Ibn Maalik and Abu Al-‘Aaliyah said: “They used to pray between Maghrib and 'Ishaa'."

In any case, a Muslim should be moderate as stated in a Prophetic Hadeeth: "Try to be near perfection and receive glad tidings that you will be rewarded; gain strength by offering the prayers in the mornings, afternoons and during the last hours of the nights; you will reach your intended purpose."

Allaah Knows best.