Mozart's variations on 'Je suis Lindor' are a delightful display of his mastery in transforming a simple operatic aria into a virtuosic showpiece. They are characterized by their wit, charm, and ingenious melodic and harmonic inventiveness.
Focus on achieving fluid arpeggiated passages and precise articulation in the faster sections.
12 Variations on 'Je suis Lindor' (K.354) 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 12 Variations on 'Je suis Lindor' (K.354) lasts about 13 minutes.
12 Variations on 'Je suis Lindor' (K.354) is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.