The time for Suhoor is between the middle of the night and dawn. It is considered best to delay it (that is, as close to dawn a possible).
The Prophet (S) said: "Hasten the breaking of the fast and delay the Suhoor". [Sahih al-Jami`]
Zaid ibn Thabit reported: "We ate Suhoor with the Messenger of Allah and then we got up for the prayer. He was asked: 'What was the amount of time between the two?' He responded: '[The time it would take to recite] fifty ayat."' [al-Bukhari and Muslim].
'Amr ibn Maimun adds: "The companions of Muhammad (S), would be the first to break the fast and the last to eat their Suhoor." [al-Baihaqi, Abdur-Razzaq and al-Haithami; Sahih]
The Prophet (S) said: "We, the Prophets, have been ordered to hasten breaking the fast, delay the Suhoor, and to put our right hand on the left one in prayer". [Sahih al-Jami`]