Raff's 'Am Gießbach' is a quintessential Romantic character piece imitating the sounds of a cascading stream. It is distinctive for its flowing, constant triplet movement.
Keep the constant flowing motion even while utilizing rubato.
Am Gießsbach (Op.88) is rated Henle level 4 of 9 — intermediate repertoire, roughly ABRSM grades 5–6. In the RCM system it corresponds to about level 7.
You should be comfortable at Henle level 4 — meaning you can already play level 3–4 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 Am Gießsbach (Op.88) lasts about 4 minutes.
Am Gießsbach (Op.88) is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.