Alkan's 'Etudes in All the Major Keys' are monumental works that explore a vast range of technical and musical challenges. They are known for their immense difficulty and intellectual rigor.
These etudes present extreme technical challenges, often requiring colossal strength and stamina.
12 Etudes in All the Major Keys (Op.35) is rated Henle level 8 of 9 — concert repertoire, roughly diploma level. In the RCM system it corresponds to about level 9.
You should be comfortable at Henle level 8 — meaning you can already play level 7–8 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 Major Keys (Op.35) lasts about 50 minutes.
12 Etudes in All the Major Keys (Op.35) is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.