Zaid bin Thabit said, “We took the “Suhur” (the meal taken before dawn while fasting is observed) with the Prophet and then stood up for the (morning) prayer.” I asked him how long the interval between the two (Suhur and prayer) was. He replied, ‘The interval between the two was just sufficient to recite fifty to Sixth ‘Ayat.”
So around 10-15 minutes?
Hadith no. 550 as above.
Narrated Sahl bin Sa’d:
I used to take the “Suhur” meal with my family and hasten so as to catch the Fajr (morning prayer) with Allah’s Apostle.
The believing women covered with their veiling sheets used to attend the Fajr prayer with Allah’s Apostle, and after finishing the prayer they would return to their home and nobody could recognize them because of darkness.
So Fajr starts right after Suhoor. And its time ends when darkness is no more (right before sunrise).