The Day of Judgment (Qiyamah) begins with the people leaving their graves (Resurrection), and lasts until the people of Paradise enter Paradise and the people of Hellfire enter Hellfire. The length of this Day is 50,000 years of what we count. Allah ta^ala said in Surat al-Ma^arij verse #4:
﴿في يوم كان مقداره خمسين ألف سنة﴾ سورة المعارج
The verse means: [During a Day whose length is 50,000 years].