Praise be to Allah.
The time for ‘Isha’ prayer lasts from when the red afterglow in the sky after sunset disappears until halfway through the night.
It is preferable to delay ‘Isha’ prayer so long as that does not cause any hardship to the people, because Abu Hurayrah (may Allaah be pleased with him) narrated that the Messenger of Allaah (peace and blessings of Allaah be upon him) said: “Were it not that it would be too hard for my ummah, I would have commanded them to delay ‘Isha’ until one-third of the night had passed, or one half.” Narrated by al-Tirmidhi, 167.
This hadeeth indicates that it is mustahabb to delay ‘Isha’ so long as that does not cause hardship to the members of the congregation. If it will cause hardship to them, then the prayer should be brought forward.
It was narrated that ‘Aa’ishah (may Allaah be pleased with her) said: The Prophet (peace and blessings of Allaah be upon him) stayed late one night until most of the night had gone and the people in the mosque had fallen asleep, then he went out and prayed. He said, “This would be its time, were it not that this would be difficult for my ummah.” Narrated by Muslim, 638.
And it was narrated that Jaabir said, when he mentioned the times when the Prophet (peace and blessings of Allaah be upon him) prayed: “He sometimes delayed ‘Isha’, and sometimes brought it forward if he saw that they had gathered early. But if he saw that they were coming late, he would delay it.” Narrated by al-Bukhaari, 1/141; Muslim, 646. See Ma’rifat Awqaat al-‘Ibaadaat by Dr. Khaalid al-Mushayqih, 1/291.
In some countries the people are accustomed to delaying ‘Isha’ for half an hour or so after the time for it begins, so that people can break their fast at leisure and get ready to pray ‘Isha’ and Taraaweeh.
There is nothing wrong with doing this, on condition that the imam does not delay the prayer so much that he causes hardship to the members of the congregation, as mentioned above.
It is better to refer to the people who attend the mosque and reach an agreement with them on the time for the prayer, because they know best what suits them.
And Allaah knows best.