Cramer's 'Melodious Preludes' are studies in developing both technique and musicality. They are known for their elegant melodies and effective preparation for more complex repertoire.
Focusing on legato phrasing and evenness in the rapid passagework is crucial.
36 Préludes mélodiques (Op.91) is rated Henle level 4 of 9 — intermediate repertoire, roughly ABRSM grades 5–6. In the RCM system it corresponds to about level 6.
You should be comfortable at Henle level 4 — meaning you can already play level 3–4 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 36 Préludes mélodiques (Op.91) lasts about 40 minutes.
36 Préludes mélodiques (Op.91) is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.