This 'Allegro assai in B-flat major' by Mozart is a spirited and graceful movement, showcasing his characteristic clarity and melodic invention. It is a fine example of Classical sonata form with its balanced phrasing and lively figuration.
Develop evenness of touch and articulate the rapid scalar passages with precision.
Allegro assai in B-flat major 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 Allegro assai in B-flat major lasts about 4 minutes.
Allegro assai in B-flat major is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.