Sehri time expires at the time of Subah Sadiq. Subah Sadiq (Pre dawn) is when the time of Fajr enters. Sehri ends at the time of Subah Sadiq. As a precaution all eating must stop at least 5 minutes before the time of Subah Sadiq.

Warning.

Many people continue to eat right to the time of Fajr Azaan. Fajr Azaan generally takes place approximately 5 to 10 minutes after Subah Sadiq. Thus a person having eaten up to this time would have eaten during the time of fasting. In such a case the fast will be invalid.

Courtesy: www.everymuslim.net