Based on my estimates, it may take around 2-3 hours to complete the Puhua An River scenic area. Considering you’ll likely want some time for lunch and then head back after finishing your meal around noon, I’d say finishing by late afternoon or early evening is realistic.
Going there will depend on various factors such as traffic conditions, how much walking/riding you plan to do within the scenic area, and of course, your personal pace. However, it shouldn’t be too rushed if you leave around midday with some buffer time in case things take longer than expected.