I think the longest song I have in my collection is the Great Barrier Reefer by Bongripper, which is 1 hour, 19 minutes, and 35 seconds.
The second longest is most likely Dopesmoker by Sleep, which is 1 hour, 13 minutes, and 7 seconds.
I believe the third longest I have is Absolutego by Boris, which is 1 hour, 5 minutes, and 35 seconds.
The fourth longest would probably be Thaumogenesis by Nadja, which is 1 hour, 1 minute, and 43 seconds.
I think the fifth longest is Monolithe I by Monolithe, which is 51 minutes and 58 seconds.