How much did the Prophet (blessings and peace of Allah be upon him) recite in the obligatory prayers?

The Prophet (blessings and peace of Allah be upon him) used to recite at length in Fajr and Zuhr prayer, a moderate amount in ‘Asr and ‘Isha’, and briefly in Maghrib, in which he would recite short soorahs. 

He (blessings and peace of Allah be upon him) would sometimes recite longer passages than this or he would make it shorter, according to the situation. 

It was narrated from Sulaymaan ibn Yasaar from Abu Hurayrah (may Allah be pleased with him) who said: I never prayed behind anyone whose prayer was more like that of the Messenger of Allah (blessings and peace of Allah be upon him) than So and so. Sulaymaan said: He would make the first two rak‘ahs of Zuhr lengthy, and the last two shorter; he would make ‘Asr brief; in Maghrib he would recite the short soorahs from al-mufassal; in ‘Isha’ he would recite medium-length soorahs from al-mufassal; and in Fajr he would recite long soorahs from al-mufassal.

Narrated by an-Nasaa’i (972); classed as saheeh by al-Albaani in Saheeh Sunan an-Nasaa’i. 

Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said: 

The lengthy soorahs of al-mufassal are from Qaaf (50) to an-Naba’ (78); from an-Naba’ to ad-Duha (93) they are medium length; and from ad-Duha to the end they are short.

End quote from ash-Sharh al-Mumti‘, 3/75 

Ibn Battaal (may Allah have mercy on him) said: The scholars are unanimously agreed that the longest of the prayers is the recitation in Fajr… Then he quoted a number of reports from the Sahaabah (may Allah be pleased with them)… Then he said: This difference of opinion among the early generation indicates that they understood from the Messenger that it is permissible to make the recitation in Fajr prayer long or short, and that there is no set limit in that beyond which it is not permissible to go. It may be, and Allah knows best, that those among the Sahaabah who recited at length knew that the people praying behind them were eager to make the prayer lengthy. But nowadays it is more appropriate to make the prayer brief, because among the people are those who are sick or elderly or who need to go about their business, as the Prophet (blessings and peace of Allah be upon him) said to Mu‘aadh and as Maalik said there may be a man who wants to go back to his work or who may be needed to help others when he is praying Fajr and Zuhr, so it is recommended for the imam to recite a short soorah. The same applies to the traveller; people praying behind him may want him to keep it brief.

End quote from Sharh Saheeh al-Bukhaari, 2/385 

Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said: 

It is best for the imam to seek to emulate the prayer of the Messenger of Allah (blessings and peace of Allah be upon him) and the way in which he led his Companions in prayer. Rather this is what is prescribed and is what the Imams enjoined, as it is proven in as-Saheeh that the Prophet (blessings and peace of Allah be upon him) said to Maalik ibn al-Huwayrith and his companion: “When the time for prayer comes, give the adhaan (call to prayer) and the iqaamah (call immediately preceding the prayer), let one of you lead the other in prayer and pray as you have seen me praying.” And it is proven in as-Saheeh that he used to recite in Fajr prayer something between sixty and one hundred verses, which is between approximately one third and one half of a juz’, of which there are thirty. So he would recite the lengthy soorahs of al-mufassal such as Qaaf (50), “Alif-Laam-Meem tanzeel”, and al-Mulk (67); and he would recite Soorat al-Mu’minoon (23), as-Saaffaat (37) and so on… And sometimes he would make it brief, either because he was travelling or for some other reason, as he (blessings and peace of Allah be upon him) said: “I may start to pray, intending to make it lengthy, then I hear the crying of a child so I make it brief because of what I know of his mother’s concern for him.” And it was narrated from him that he used to recite in Fajr Soorat at-Takweer (81) and Soorat az-Zalzalah (99). So the imam should usually do what the Prophet (blessings and peace of Allah be upon him) usually used to do, then if there is a purpose to be served by making it longer than that or shorter than that, then he should do so, as the Prophet (blessings and peace of Allah be upon him) sometimes used to do more than that or less than that. 

End quote from Majmoo‘ al-Fataawa, 22/318

And Allah knows best.