It's been a while since I've seen comme_le_vent post here, now that you mention it. Anyway, as stevie says, the entire Op.39 set is somewhat over two hours, and it breaks down as follows:
1. Comme le vent, A minor - about four and a half minutes if played at the prescribed tempo, but actually playing it at that speed is about as feasible as playing the first movement of the Hammerklavier at the tempo indicated by Beethoven - it usually takes about five minutes.
2. En rhythme molossique, D minor - seven and a half minutes or so. This one is actually not too difficult (not too easy either), and has a haunting conclusion.
3. Scherzo diabolico, G minor - four and a half minutes or so. Your hands really need to fly up and down the register in this one.
4. Symphony, first movement, C minor - anywhere between ten and fourteen minutes (Ringeissen takes fourteen - not sure about Hamelin, Smith, or Gibbons). Also has a haunting conclusion.
5. Symphony, second movement, Marche funebre, F minor - seven minutes or so. The easiest of the lot, though it requires considerable finger independence in both hands (legato and staccato side by side, for example).
6. Symphony, third movement, Menuet, B-flat minor - five and a half to six minutes. Also not too difficult if you have good octave technique.
7. Symphony, fourth movement, Finale, E-flat minor - four and a half to five minutes. Not nicknamed "a ride through hell" for nothing!
8. Concerto, first movement, G-sharp minor - thirty minutes, give or take a few (Hamelin takes twenty-eight), and over a thousand bars long. If "Comme le vent" isn't the most difficult, this one is if only for its sheer length and emotional spectrum (but there is plenty of good old-fashioned technical difficulty to be found as well).
9. Concerto, second movement, C-sharp minor - twelve or thirteen minutes. Easiest movement of the concerto but still not to be taken lightly.
10. Concerto, third movement, F-sharp minor - nine minutes. And a harrowing nine minutes it is, but what a conclusion!
11. Overture, B minor - twelve to fourteen minutes. Among the other minor key etudes, its difficulty is middling, but it's certainly exhausting to play with its repeated note introduction and numerous double octave passages.
12. Le festin d'Esope, E minor - nine minutes. What a ride it is, as well - I wonder if anyone has ever sat down and successfully (or nearly so) attached each variation to a particular animal?
Of those, I can at least fumble through Nos.2, 5, 6, 9, and 11 on a good day, but the others are utterly beyond me. Their reputation for difficulty has scared off many performers over the years, but with Alkan experiencing a resurgence in popularity it's rather easier to find recordings now. I have Bernard Ringeissen's recordings of Nos.1-7, 11, and 12 as well as the complete Op.35 (these twenty-one are available on two Naxos/Marco Polo discs) and Hamelin's rendition of the Concerto, but there are also recordings by the late Ronald Smith and Jack Gibbons of the complete set, while Hamelin has recorded the Symphony and Le Festin d'Esope (the latter is on his brilliant disc with the Grande sonate and Sonatine in A minor). I've never heard Smith or Gibbons' recordings but they come highly recommended.