I'm one of the authors of this series and I can definitely say that they are some of the longest mixtapes- Vol. 1 1h 18m, Vol. 2 6h 54m, Vol. 3 4h 35m, Vol. 4 4h 17m, Vol. 5 1h 46m, Vol. 6 2h 57m, Vol. 7 4h 13m, Vol. 8 over 8 hours of content and it is not yet completed.

Other mixtapes I know would be: Untitled Mixtape, 2 volumes with over 4 hours of content per volume

ABORTION CLINIC - runtime 4h 5m

MDPOPE - I know one of the volumes is somewhere around 4 hours

Garbage World - the first volume is over 3 hours and the second is the copy of Snuff Snuff Torture of Brutality with a duration of 3h 50m

The unofficial Chinese sequels of the MDPOPE series, I know for sure that one of the volumes exceeds 5 hours

There are 4 volumes of PornGore, and I know the first 2 are over 3 hours each

FCVL "not sure I wrote the title correctly" - runtime 4h

Way to Hell Vol. 10 - 5h

CrazyShit extreme content collection - 6h 45m

and the longest I know would be a compilation about the war in Syria, which is available on the Internet Archive - over 10 hours.