What is correct it to make it neither too long nor too short. Making it so long that it causes hardship to people is not allowed. When the Prophet (peace and blessings of Allaah be upon him) heard that Mu’aadh ibn Jabal was making the prayers too long for his people, he spoke to him angrily in a manner that he had never used when advising someone and said to Mu’aadh: “O Mu’aadh, are you putting people off?” (al-Bukhaari, al-Adab, 6106; Muslim, al-Salaah, 465). What the imam should do is to adhere to the Sunnah.
Undoubtedly making the du’aa’ too long causes hardship to the people and exhausts them, especially those who are weak. Some people may have work to do, or they may not want to finish before the imam but it is difficult for them to stay with the imam. So we advise our brother imams to follow a moderate course, and to omit the du’aa’ sometimes so that the people will not think that the du’aa’ is obligatory.