This is a chorale prelude by J.S. Bach, based on the hymn tune of the same name. It is characterized by its rich contrapuntal texture and moving harmonic progressions.
Mastering the independent voice leading and smooth contrapuntal lines is crucial.
Bleib bei uns, denn es will Abend werden (BWV 6) is rated Henle level 6 of 9 — advanced repertoire, roughly ABRSM grades 7–8. In the RCM system it corresponds to about level 8.
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 Bleib bei uns, denn es will Abend werden (BWV 6) lasts about 5 minutes.
Bleib bei uns, denn es will Abend werden (BWV 6) is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.