Schubert's 'Adagio' in E major, D.612, is a tender and lyrical piece, showcasing a simple yet beautiful melody with subtle harmonic color.
The primary challenge lies in maintaining a delicate and singing tone while executing the expressive rubato.
Adagio in E (D.612) is rated Henle level 5 of 9 — late intermediate repertoire, roughly ABRSM grades 6–7. In the RCM system it corresponds to about level 8.
You should be comfortable at Henle level 5 — meaning you can already play level 4–5 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 Adagio in E (D.612) lasts about 4 minutes.
Adagio in E (D.612) is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.