It is important to note that it is obligatory to recite al-Fātiḥah in every single rak'ah (unit) of the prayer.

Abu Hurayrah (may Allah be pleased with him) narrated that the Prophet (may the peace and blessings of Allah be upon him) said:

"Whoever prays without reciting al-Fātiḥah, then his prayer is incomplete." He repeated it three times. so it was said to Abu Hurayrah, "What if we pray behind the imam?" He said, "Recite it to yourself" [Muslim: 395]

Jābir ibn 'Abdullāh (may Allah be pleased with him) narrated: 

"We used to recite behind the imam in dhuhr and 'asr [prayers] - Sūrah al-Fātiḥah and another chapter in the first two rak'ah, and only Sūrah al-Fātiḥah in the last two rak'ah."  [Ibn Mājah: 843]

The Prophet (may the peace and blessings of Allah be upon him) said: 

"Then recite Sūrah al-Fātiḥah, and [after that recite] whatever (chapter) you like." [Abū Dāwood: 859]

The imaam should not make his recitation very long, especially when it is difficult for those who may be praying behind him, such as the elderly, the sick, nursing mothers, or those who have other engagements. It was narrated that Mu'ādh ibn Jabal used to pray the 'Ishā prayer with the Prophet (may the peace and blessings of Allah be upon him) and after that he would go and lead his people in prayer. Once he led his people in the 'Ishā prayer and he prolonged the recitation so much, that a man left the prayer, and complained to the Prophet (may the peace and blessings of Allah be upon him). The Prophet (may the peace and blessings of Allah be upon him) said to Mu'ādh (may Allah be pleased with him):

"Are you putting people to trial!?"  repeating this three times. "Why not read [Surah Al-A’laa], [Ash-Shams] and [Al-Layl]? For certainly, behind you prays the elderly, the weak and the infirm." [al-Bukhārī 705]