When does the time for the 'Isha prayer end, according to the four Imams, and what is the most preponderant opinion on this?

The time for the 'Isha prayer extends until the appearance of true dawn, according to the schools of the four Imams, and this is the most preponderant opinion. It is preferable not to delay it beyond the first third of the night. However, if it is delayed until the middle of the night, it is permissible. What is after the middle of the night is considered a time of necessity.