I want to ask that till what time we are allowed to eat sehri either till the time of imsak or till the time of Fajr azan?
You can eat sehri till true dawn appears. However it is better to stop eating before some time as precaution. Azan is called after true dawn hence it is not lawful to eat till azan.
Allah (Subhana Wa Ta’ala) knows Best
Darul Uloom Deoband