The Well-Tempered Clavier is a monumental collection of preludes and fugues in all major and minor keys, showcasing Bach's mastery of contrapuntal writing and harmonic exploration. Each pair is a distinct musical argument, demonstrating immense variety within a strict framework.
Mastering the intricate counterpoint and achieving clarity in all voices is the primary challenge.
6 Präludien und Fugen is rated Henle level 7 of 9 — late advanced repertoire, roughly ABRSM grade 8 and above.
You should be comfortable at Henle level 7 — meaning you can already play level 6–7 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 6 Präludien und Fugen lasts about 90 minutes.
6 Präludien und Fugen is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.