Short answer
From true dawn until sunset, every day of the month. The fast starts at true dawn, the beginning of Fajr time, which comes well before sunrise, and ends at sunset. Ramadan lasts 29 or 30 days, and the length of each fasting day depends on where you live and the season.
The fast runs from the rising of true dawn (subh sadiq), the start of Fajr time, until sunset. Eating is allowed right up to true dawn, and delaying suhoor close to it is sunnah, but stop a few minutes early to be safe: a fast in which you ate after dawn has to be made up.
The four schools agree on this.
Explanation
Allah says: eat and drink until the white thread of dawn becomes distinct to you from the black thread of night, then complete the fast until the sunset. The white thread is true dawn (subh sadiq): the light that spreads along the horizon and marks the start of Fajr time. It is not sunrise. Sunrise comes later, at the end of Fajr time.
A common mistake. Some people new to Ramadan think they can eat until the sun comes up. They cannot. Once Fajr time begins, the fast has begun, even if you have not prayed Fajr yet. Many timetables print an imsak time a few minutes before Fajr. It is a safety margin, not the real start, but do not leave your last sip that late on purpose.
How many hours? It depends on your location and the time of year. Ramadan follows the moon, so it comes about eleven days earlier each year, and the fasting day is longer in summer and shorter in winter. For places where the day is very long, see fasting when the days are very long.
How many days? An Islamic month has 29 or 30 days. The Prophet ﷺ showed the month with his hands, meaning sometimes 29 days and sometimes 30. Ramadan begins with the sighting of the new moon and ends when the moon of Shawwal is seen, or after 30 days.
From sunset until true dawn you may eat, drink and be with your spouse. The four schools agree on these limits. For the meal before dawn, see the suhoor guide.
The evidence
Quran, Surah Al-Baqara 2:187
أُحِلَّ لَكُمْ لَيْلَةَ ٱلصِّيَامِ ٱلرَّفَثُ إِلَىٰ نِسَآئِكُمْ ۚ هُنَّ لِبَاسٌۭ لَّكُمْ وَأَنتُمْ لِبَاسٌۭ لَّهُنَّ ۗ عَلِمَ ٱللَّهُ أَنَّكُمْ كُنتُمْ تَخْتَانُونَ أَنفُسَكُمْ فَتَابَ عَلَيْكُمْ وَعَفَا عَنكُمْ ۖ فَٱلْـَٰٔنَ بَٰشِرُوهُنَّ وَٱبْتَغُوا۟ مَا كَتَبَ ٱللَّهُ لَكُمْ ۚ وَكُلُوا۟ وَٱشْرَبُوا۟ حَتَّىٰ يَتَبَيَّنَ لَكُمُ ٱلْخَيْطُ ٱلْأَبْيَضُ مِنَ ٱلْخَيْطِ ٱلْأَسْوَدِ مِنَ ٱلْفَجْرِ ۖ ثُمَّ أَتِمُّوا۟ ٱلصِّيَامَ إِلَى ٱلَّيْلِ ۚ وَلَا تُبَٰشِرُوهُنَّ وَأَنتُمْ عَٰكِفُونَ فِى ٱلْمَسَٰجِدِ ۗ تِلْكَ حُدُودُ ٱللَّهِ فَلَا تَقْرَبُوهَا ۗ كَذَٰلِكَ يُبَيِّنُ ٱللَّهُ ءَايَٰتِهِۦ لِلنَّاسِ لَعَلَّهُمْ يَتَّقُونَ ﴿١٨٧﴾
It has been made permissible for you the night preceding fasting to go to your wives [for sexual relations]. They are clothing for you and you are clothing for them. Allah knows that you used to deceive yourselves, so He accepted your repentance and forgave you. So now, have relations with them and seek that which Allah has decreed for you. And eat and drink until the white thread of dawn becomes distinct to you from the black thread [of night]. Then complete the fast until the sunset. And do not have relations with them as long as you are staying for worship in the mosques. These are the limits [set by] Allah, so do not approach them. Thus does Allah make clear His ordinances to the people that they may become righteous.
The fast begins at true dawn and ends at sunset.
Sahih al-Bukhari 617
Sahih... إن بلالا يؤذن بليل فكلوا واشربوا حتى ينادي ابن أم مكتوم ...
"Bilal calls the adhan while it is still night, so eat and drink until Ibn Umm Maktum calls it."
The limit for eating is true dawn itself, not an early call before it.
Sahih al-Bukhari 1913
Sahih... إنا أمة أمية لا نكتب ولا نحسب الشهر هكذا وهكذا ...
"We are an unlettered nation; we do not write or calculate. The month is like this and like this."
A lunar month is 29 or 30 days, so Ramadan has 29 or 30 fasts.
Where the ruling is recorded
- al-Hidaya, Kitab al-Sawm, opening chapter. The fast lasts from the rising of the second (true) dawn until sunset.
- Radd al-Muhtar (Ibn Abidin), Kitab al-Sawm, chapter on what is disliked and recommended for the fasting person. Delaying suhoor is recommended as long as one is not in doubt about dawn.
Checked by our team. For a more accurate answer to your own situation, consult a scholar. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.