Short answer
The Quran describes it as a Day "the extent of which is fifty thousand years" (70:4), and a sahih hadith uses the same measure. But it will not feel the same for everyone: it is "a difficult day, for the disbelievers, not easy" (74:9-10), while Allah makes it lighter for the believers.
Belief in the Day as Allah describes it is required. Its length in our measures is part of the unseen, so we affirm what the texts say without going beyond them.
The four schools agree on this.
Explanation
Surah al-Ma'arij says: "The angels and the Spirit will ascend to Him during a Day the extent of which is fifty thousand years" (70:4). Many commentators, including reports from Ibn Abbas, apply this to the Day of Resurrection. The Prophet ﷺ confirmed the measure when he described the punishment of someone who did not pay zakah on his wealth: it will go on "in a day whose length is fifty thousand years" until judgement is made between the servants (Sahih Muslim).
Other verses mention a thousand years: "a day with your Lord is like a thousand years of those which you count" (22:47), and a day of a thousand years in which the affair ascends to Him (32:5). Scholars explain that these describe different matters or stages, and that time in the Hereafter is not like our time.
Its length is not felt equally. The Quran says, "That Day will be a difficult day, for the disbelievers, not easy" (74:9-10). For the righteous it is lightened. The people of Paradise will have "a better settlement and better resting place" that Day (25:24), and some commentators took this to mean their reckoning ends before midday. The Prophet ﷺ also named seven whom Allah will shade on a day when there is no shade but His, such as a just ruler and a young man who grew up worshipping Allah.
So the useful question is not only how long, but how you will spend it. Read What happens on the Day of Judgement? and Is the Day of Judgement near?
The evidence
Quran, Surah Al-Ma'aarij 70:4
تَعْرُجُ ٱلْمَلَٰٓئِكَةُ وَٱلرُّوحُ إِلَيْهِ فِى يَوْمٍۢ كَانَ مِقْدَارُهُۥ خَمْسِينَ أَلْفَ سَنَةٍۢ ﴿٤﴾
The angels and the Spirit will ascend to Him during a Day the extent of which is fifty thousand years.
Gives the length of the Day.
Quran, Surah Al-Muddaththir 74:9-10
فَذَٰلِكَ يَوْمَئِذٍۢ يَوْمٌ عَسِيرٌ ﴿٩﴾ عَلَى ٱلْكَٰفِرِينَ غَيْرُ يَسِيرٍۢ ﴿١٠﴾
That Day will be a difficult day For the disbelievers - not easy.
The Day is hard for disbelievers and not for believers.
Quran, Surah Al-Furqaan 25:24
أَصْحَٰبُ ٱلْجَنَّةِ يَوْمَئِذٍ خَيْرٌۭ مُّسْتَقَرًّۭا وَأَحْسَنُ مَقِيلًۭا ﴿٢٤﴾
The companions of Paradise, that Day, are [in] a better settlement and better resting place.
The people of Paradise rest well on that Day.
Sahih Muslim 987
Sahih... في يوم كان مقداره خمسين ألف سنة ...
"In a day whose length is fifty thousand years."
Confirms the measure of the Day.
Sahih al-Bukhari 660
Sahih... سبعة يظلهم الله في ظله يوم لا ظل إلا ظله ...
"Seven whom Allah will shade in His shade on the day when there is no shade but His."
Some believers will be protected from its hardship.
Where the ruling is recorded
- Tafsir Ibn Kathir, Commentary on Surah al-Ma'arij, verse 4. Sets out the views on which day the fifty thousand years refers to.
- al-Aqida al-Tahawiyya, Section on the resurrection. Affirms the Day of Resurrection and the reckoning.
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.