Answer
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
The time of Isha prayer begins when the red twilight disappears and continues up to half of the night. As for the allowable time and the time due to need, it lasts until dawn.
In his famous work, Fiqh As-Sunnah, Sheikh Sayyid Sabiq, elaborates on the subject as follows:
Time of Isha prayer
The time of Isha prayer begins when the red twilight disappears and continues up to half of the night. Aishah (may Allah be pleased with her) reported: “They used to pray the night (Isha) prayer between the disappearance of the twilight and the final third of the night’s beginning.” (Al-Bukhari)
Abu Hurairah may Allah be pleased with him) reported that the Messenger of Allah (peace be upon him) said, “If it were not to be a hardship upon my nation, I would order them to delay the night prayer until a third or a half of the night had passed.” (Related by Ahmad, Ibn Majah and at-Tirmidhi, who said it is sahih.)
Abu Sa`eed Al-Khuri may Allah be pleased with him), “Once, we waited for the Messenger of Allah to lead the Isha prayer until half the night had passed, at which time he came and prayed with us. He said, ‘Stay in your places of sitting while the people have gone to their places of lying down (for sleep), for you are in prayer as long as you are waiting for the prayer. If it were not for the weakness of the weak, the illness of the ill and the need of those who have needs, I would have delayed the time of this prayer to a half of the night.” (Reported by Ahmad, Abu Dawud, Ibn Majah, an-Nasa’i and Ibn Khuzaimah. Its chain is sahih).
The hadith describes the best time to pray.
As for the allowable time and the time due to need, it lasts until dawn.
Abu Qatadah (may Allah be pleased with him) reported that the Messenger of Allah (peace be upon him) said, “There is no negligence in sleeping, but the negligence lies in not praying a prayer until the time of the next prayer has come.” (Related by Muslim)
This hadith shows that the time of every prayer continues until the beginning of the time for the next prayer, except for the morning prayer, as all scholars agree that its time lasts only until sunrise.
Can you delay Isha Prayer?
It is most virtuous to delay the night prayer until the end of the preferred time for it, which is half the night. Aishah reported, “One night the Prophet (may Allah be pleased with her) prayed the Isha prayer after most of the night had gone and most of the people in the mosque had fallen asleep. Then he came out, prayed, and said, “This would be the proper time if it were not a hardship on my nation.”‘ (Related by Muslim and an-Nasa’i)
The Prophet (peace be upon him) did not do this on a regular basis, as he heard that it would be a hardship on his nation. He would take into consideration the situation of those in the mosque. Sometimes, he would hasten in performing the prayer and at other times he would delay it.
Jabir (may Allah be pleased with him) said: “The Messenger of Allah would pray the noon prayer during the hottest time of noon, the afternoon prayer when the sun was clear, the sunset prayer when the sun had gone down, and the night prayer he would sometimes delay and sometimes hasten if he found people gathered (in the mosque). If he noticed that they were lingering, he would delay it. He would pray the morning prayer while it was still dark.” (Related by al-Bukhari and Muslim)
Almighty Allah knows best.