Alkan's 'Etudes in All the Minor Keys' are renowned for their extreme difficulty and profound musical content, particularly the 'Concerto da camera' and the symphonic etude. These works represent a peak of Romantic pianism.
Demands exceptional technical command, including massive chordal passages and complex polyphonic textures.
12 Etudes in All the Minor Keys (Op.39) is rated Henle level 9 of 9 — virtuoso repertoire, roughly concert / virtuoso level. In the RCM system it corresponds to about level 10.
You should be comfortable at Henle level 9 — meaning you can already play level 8–9 pieces cleanly. Learning it one level early is possible as a stretch piece, but more than that usually leads to months of frustration.
A typical performance of 12 Etudes in All the Minor Keys (Op.39) lasts about 60 minutes.
12 Etudes in All the Minor Keys (Op.39) is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.