There is only one limit for all places, which is mentioned in the Quran that sawm should continue from appearance of true dawn to the sunset.
وَكُلُوا وَاشْرَبُوا حَتَّى يَتَبَيَّنَ لَكُمُ الْخَيْطُ الأبْيَضُ مِنَ الْخَيْطِ الأسْوَدِ مِنَ الْفَجْرِ ثُمَّ أَتِمُّوا الصِّيَامَ إِلَى اللَّيْلِ (2:187)
There may be difference as per the season and place, so if the day is 16 hour long in your country, you should keep the full day from rise of true dawn to sunset. What the student said is baseless and unauthentic.
Allah (Subhana Wa Ta’ala) knows Best
Darul Uloom Deoband