What is the point of diminishing returns on the STL refinement level in Fusion 360 on an Ender 3?
I don't know that this can be definitively answered for a specific printer and all arbitrary designs.

The refinement level basically determines how smooth a curved surface will turn out.  The STL file format can only express an object in terms of triangular-shaped surfaces, so Fusion 360 will need to approximate a curved surface by breaking it up into triangles. Flat surfaces and straight edges can be represented perfectly, so they won't be affected. Low refinement will use a small number of relatively large triangles. On a part like your example, the cylindrical shaft will have noticeable facets. Higher refinement means a larger number of smaller triangles.

If you have "Preview Mesh" checked as shown, you will be able to see the triangle wireframe, and you can use your own judgment if it's "good enough".

Ultimately, higher refinement means longer processing times and larger file sizes. The final print time won't be affected much if any.

Personally, I always use high refinement. Even on my modest system, it only takes a few more seconds to prepare a multi-hour print, and maybe a few hundred kilobytes or a couple megabytes on my hard drive that I will barely notice. This is a small tradeoff to ensure the best possible STL definition.