Hadith: We had Suhūr with the Messenger (may Allah's peace and blessings be upon him), and then he stood for prayer. I asked: What was the interval between Suhūr and the Adhān? He said: As long as the recitation of 50 verses

عَنْ أَنَسٍ عَنْ زَيْدِ بْنِ ثَابِتٍ رَضِيَ اللَّهُ عَنْهُ قَالَ:

تَسَحَّرْنَا مَعَ النَّبِيِّ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ، ثُمَّ قَامَ إِلَى الصَّلاةِ، قُلْتُ: كَمْ كَانَ بَيْنَ الأَذَانِ وَالسَّحُورِ؟ قَالَ: قَدْرُ خَمْسِينَ آيَةً.
[صحيح] - [متفق عليه]

Anas ibn Mālik reported Zayd ibn Thābit said: "We had Suhūr with the Messenger (may Allah's peace and blessings be upon him), and then he stood for prayer." I asked: "What was the interval between Suhūr and the Adhān?" He said: "As long as the recitation of 50 verses."
Sahih/Authentic. - [Al-Bukhari and Muslim]

Explanation

Zayd ibn Thābit relates in this Hadīth that after the Prophet (may Allah's peace and blessings be upon him) had got Suhūr, he stood for Fajr prayer. So, Anas asked him about the duration between Suhūr and prayer, and he replied that it was as long as the recitation of 50 verses. It appears that this estimate relates to the verses moderate in their length, not very long like the verses at the end of Sūrat al-Baqarah and the beginning of Sūrat al-Mā'idah, nor very short like the verses in the Sūrahs of Ash-Shu‘arā', As-Sāffāt, al-Wāqi‘ah.