Schubert's "An die untergehende Sonne" D. 457 is a setting of a poem that contemplates the setting sun, likely evoking a somber and reflective mood. The piano part would support this with rich harmonies and a flowing, yet poignant, melodic line.
The challenge lies in shaping the melodic phrases with a deep sense of introspection and managing the harmonic colors to create a twilight atmosphere.
An die untergehende Sonne (D.457) 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 An die untergehende Sonne (D.457) lasts about 4 minutes.
An die untergehende Sonne (D.457) is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.