All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
It is difficult to specify the length of the period that is between Maghrib and Isha because the difference of that period varies according to the seasons.
What should be considered in this issue is the Sharee’ah rule which states that the end of the preferred fixed time of Maghrib is the disappearance of the twilight and this time is also the beginning of the fixed time of ‘Isha.
This specification is more accurate. This is about the fixed time of ‘Isha prayer.
As for performing the prayer itself, it should initially be done in the mosque and in congregation be that in the early time of the prayer or just before the end of its fixed time depending on the situation of the praying person. But if the person has a sound excuse to miss the prayer, then it is lawful for him to perform the prayer alone in the early or the last part of the prayer time.
It should be noted here that it is unlawful to delay the prayer except for a legal excuse like sleeping, forgetting, …etc.
Allah knows best.