Answered by Shaykh Faraz Rabbani
When does the time for maghrib expire? Is it when Isha comes in?
Maghrib time ends when Isha time comes in. However, it is disliked to unduly delay Maghrib prayer (within its time) without a genuine excuse, because the sunna of the Prophet (peace and blessings be upon him) was to perform Maghrib prayer after the time entered, without delay. [Shurunbulali, Maraqi al-Falah; Ibn al-Humam/Marhinani, Fath al-Qadir `ala al-Hidaya]