ثُمَّ إِنَّ مَرْجِعَهُمْ لَإِلَى الْجَحِيمِ ﴿٦٨﴾

Then thereafter, verily, their return is to the flaming fire of Hell.

means, after that interval, they will be sent back to the burning fire, searing heat and scorching flames, and they will rotate between the one and the other.

This is like the Ayah,

يَطُوفُونَ بَيْنَهَا وَبَيْنَ حَمِيمٍ ءَانٍ

They will go between it (Hell) and the fierce boiling water! (55:44)

Qatadah recited this Ayah when discussing this Ayah. This is a good interpretation.

Abdullah bin Mas`ud recited it differently, with the meaning

"Their return in the afternoon.''

Abdullah, may Allah be pleased with him, used to say:

"By the One in Whose Hand is my soul, midday on the Day of Resurrection will not come until the people of Paradise will be in Paradise and the people of Hell will be in Hell.''

Then he recited:

أَصْحَـبُ الْجَنَّةِ يَوْمَئِذٍ خَيْرٌ مُّسْتَقَرّاً وَأَحْسَنُ مَقِيلاً

The dwellers of Paradise will, on that Day, have the best abode, and have the fairest of places for repose. (25:24)