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.