The rendering time for that Japanes illustration seems very unusually long. I'd be interested to know what your system specs are like. Beautiful design though. I also do 3d modeling and rendering. I do motion graphics and animations mostly these days, but I have also done still images for many years. I specialize in modeling with 3dsmax, but have also learned Zbrush and Blender over the years. Most of the animations I do nowadays are created with 3dsmax and rendered with Blender. I don't think I've had a frame which took longer than 5 minutes to render in many years, but I usually render with GPU now rather than CPU. It's about 20x faster for the same quality of scene.