A virtuosic operatic fantasy on Bellini's famous aria, characteristic of the 19th-century 'fantaisie' tradition. It is distinguished by its demanding cadenzas and florid bel canto embellishments.
The student must master rapid ornamental runs while keeping the underlying bel canto melody fluid and expressive.
Comparable difficulty and character — good candidates for what to learn next.
'Casta diva che inargenti' in 'Norma' (Op.61) is rated Henle level 6 of 9 — advanced repertoire, roughly ABRSM grades 7–8. In the RCM system it corresponds to about level 9.
You should be comfortable at Henle level 6 — meaning you can already play level 5–6 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 'Casta diva che inargenti' in 'Norma' (Op.61) lasts about 6 minutes.
'Casta diva che inargenti' in 'Norma' (Op.61) is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.
Pieces at a comparable difficulty and character include All'amica lontana by Adolfo Fumagalli, All'amica lontana by Adolfo Fumagalli, 16 Waltzes by Johannes Brahms — see the full similar-pieces list on this page.