The standard of what is too long or too short in prayer is what is narrated in the Sunnah and is not what people think or like
Question

Some of the people who pray behind me have complained that I stand for too long after rising from rukoo’, because I recite the whole du’aa’ narrated (in the Sunnah) after rising from rukoo’, “Rabbana wa laka’l-hamd hamdan katheeran tayyiban mubaarakan fihi…” Is there a shorter du’aa’ that I can recite after rising from rukoo’ so that this will not be so difficult for the people?
Praise be to Allah.The imaam and all those who have been appointed in charge of some work must pay attention to the Sunnah, and not give in to anyone who tells him to against the Sunnah. There is nothing wrong with shortening the prayer occasionally, if there is a need to do so, as the Prophet (peace and blessings of Allaah be upon him) used to do. But ordinarily the imam must adhere to the Sunnah. So adhere to the Sunnah and tell the people that if they are patient in doing so they will earn the reward of those who are patient in obeying Allaah. If the matter of what is too short or too long were left to people’s whims and desires, the ummah would have split into factions. What is moderate to some may be too long for others, so you should follow what is narrated in the Sunnah, which is well known, praise be to Allaah.
 Hence I advise every imaam who leads the Muslims in prayer in the mosques to strive to recite what is written by the scholars describing the prayer of the Prophet (peace and blessings of Allaah be upon him), such as Kitaab al-Salaah by Ibn al-Qayyim, which is a well-known book, and what he mentioned in Zaad al-Ma’aad fi Hadi Khayr al-‘Ibaad.