This is a powerful and deeply expressive work, originally written for musical box. The adagio section is intensely sorrowful, leading into a remarkably intricate and somber fugue that showcases Mozart's mastery of contrapuntal writing.
Developing the dramatic weight and clarity of the fugal lines while maintaining the emotional intensity is the main challenge.
Adagio and Rondo in C minor (K.617) 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 Adagio and Rondo in C minor (K.617) lasts about 7 minutes.
Adagio and Rondo in C minor (K.617) is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.