نَحْنُ أَعْلَمُ بِمَا يَقُولُونَ
We know very well what they will say,
This means in their condition of conversing amongst themselves.
إِذْ يَقُولُ أَمْثَلُهُمْ طَرِيقَةً
when the best among them in knowledge and wisdom will say;
the one with perfect intelligence amongst them,
إِن لَّبِثْتُمْ إِلاَّ يَوْمًا
You stayed no longer than a day!
This is because on the Day of Judgement they will sense the shortness of the worldly life within themselves. For the worldly life, with its repetitious time periods and successive nights, days and hours, is as if it is just one day. For this reason, on the Day of Resurrection the disbelievers will think the worldly life was very short. By this they mean to prevent the establishment of the evidence against them due to the shortness of time that they had.
Allah says about this,
وَيَوْمَ تَقُومُ السَّاعَةُ يُقْسِمُ الْمُجْرِمُونَ مَا لَبِثُواْ غَيْرَ سَاعَةٍ
And on the Day that the Hour will be established, the criminals will swear that they stayed not but an hour, until His statement,
وَلَكِنَّكُمْ كُنتُمْ لَا تَعْلَمُونَ
(but you knew not"). (30;55-56)
Allah also says,
أَوَلَمْ نُعَمِّرْكُمْ مَّا يَتَذَكَّرُ فِيهِ مَن تَذَكَّرَ وَجَأءَكُمُ النَّذِيرُ
Did We not give you lives long enough, so that whosoever would receive admonition could receive it! And the warner came to you. (35;37)
Allah, the Exalted, also says,
قَـلَ كَمْ لَبِثْتُمْ فِى الاٌّرْضِ عَدَدَ سِنِينَ
قَالُواْ لَبِثْنَا يَوْماً أَوْ بَعْضَ يَوْمٍ فَاسْأَلِ الْعَأدِّينَ
قَالَ إِن لَّبِثْتُمْ إِلاَّ قَلِيلً لَّوْ أَنَّكُمْ كُنتُمْ تَعْلَمُونَ
(Allah will say);What number of years did you stay on earth!
They will say;"We stayed a day or part of a day. Ask of those who keep account."
He (Allah) will say;"You stayed not but a little, if you had only known!" (23;112-114)
This means that you only remained in it (the earth) a little while. If you only knew, you would have preferred the eternal life over the temporal life. Yet, you conducted yourselves in an evil manner. You gave the present, temporary life precedence over the eternal and everlasting life