This Allegretto by Schubert is a powerful and brooding piece, often considered an impromptu. It features striking dynamic contrasts and a highly emotive melodic line full of yearning.
Mastering the wide dynamic range and the expressive intensity of the melodic lines is crucial.
Allegretto, in C minor 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 Allegretto, in C minor lasts about 4 minutes.
Allegretto, in C minor is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.