The angels and the Spirit (of the faithful) ascend to Him in a day the measure of which is (equal to) fifty thousand years (a day of spiritual advancement of the faithful is equivalent to fifty thousand years of material advancement, thus the development and progress of the human soul knows no end). (70:4)  
Commentary
NUMBER 50,000: In Sūrah al-Maʿārij (chapter 70), after speaking of the punishment which befell a specific person, which according to the history of revelation, was in regards to a person who mocked the Prophet’s proclamation on the day of Ghadeer of the mastership of ʿAlī b. Abī Ṭalib, Allah then states that the Angels and the Rūḥ ascend to Him in a day which is equivalent to 50,000 of “our” days. 
What does this mean? Does it really take 50,000 of “our” days for the Angels and this creation known as the Rūḥ to ascend to Allah? Is He that far away? Don’t they travel at the speed of light or something even faster? Why make mention of this number in the Quran? Is it metaphorical?
Without a doubt, this ‘ascent’ is not physical, rather, it carries with it spiritual connotations – these creations of Allah ascend to Him with haste seeking proximity to their Lord –preparing themselves to take orders from Him and then fulfill them. 
Scholars believe that the meaning of the Rūḥ in this verse is the Rūḥ al-Amīn – the great Angel also spoken about in Sūrah al-Qadr where Allah said:
تَنَزَّلُ الْمَلَائِكَةُ وَالرُّوحُ فِيهَا بِإِذْنِ رَبِّهِمْ مِنْ كُلِّ أَمْرٍ٤
And as for the phrase that this ascent takes, “…50,000 years…” to complete, well this may be an allusion to the fact that the Day of Judgement, according the days of our world and as mentioned in Sūrah al-Sajdah, verse 5, is 1,000 years and when we combine that figure with the ḥadīth which states that the Day of Judgement has 50 stations which we must traverse, doing the math of 1,000 years x 50 stations gives us the total of 50,000 years.
Other commentators of the Quran have the opinion that the usage of the term, “…50,000 years…” is metaphorical and refers to a large amount – not necessarily the actual number – but that the Day will be extremely lengthy.
In any case, as the aḥādīth tell us, the Day of Judgement and accounting on that day will be that long for the guilty ones, the oppressors and the disbelievers. 
In a ḥadīth related by Abū Saʿid al-Khudrī it has been mentioned that when this verse was revealed, someone asked the Prophet, “O Messenger of Allah! How long is that day!?” To this the Prophet replied:
وَ الَّذِى نَفْسُ مُحَمَّدٍ بِیَدِهِ إِنَّهُ لَیَخِفُّ عَلَى الْمُؤْمِنِ حَتَّى یَکُونَ أَخَفَّ عَلَیْهِ مِنْ صَلاَةِ مَکْتُوبَةِ یُصَلِّیْها فِى الدُّنْیَا
I take an oath by the One who holds the soul of Muḥammad in His hand that that Day (of judgement) will be so effortless for the true believer such that it will be less strenuous than the obligatory prayer which they prayed in the transient world.[1]
Point of Reflection
The challenges of the world to come are, without a doubt, extremely difficult and stressful, however if we maintain our akhlāq in this world, perform our obligations in terms of the aḥkām – the rules and regulations - and ensure that we have the correct ʿaqīdah – the belief system -  then with Allah’s permission, we will be able to fast-track ourselves on that day.
    
[1] Tafsīr Majmaʿ al-Bayān, v. 10, p. 353; Tafsīr al-Qurṭubī, v. 10, p. 6,761.