Schubert's Adagio et rondo concertant D.487 is a beautiful piece that contrasts a deeply expressive, lyrical Adagio with a more energetic and virtuosic Rondo.
Mastering the dynamic range and achieving a singing tone in the Adagio while executing the Rondo's rapid passages accurately is key.
Adagio et rondo concertant (D.487) is rated Henle level 5 of 9 — late intermediate repertoire, roughly ABRSM grades 6–7. In the RCM system it corresponds to about level 7.
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 et rondo concertant (D.487) lasts about 8 minutes.
Adagio et rondo concertant (D.487) is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.