one way to help Art render to get cleaner images placing lights in your scene, I don't see any lights in the bottom floor of this image, or if there is any, you should increase the intensity. The better lit your scene it will render cleaner. now, this doesn't mean you need to put thousands of lights but enough to avoid dark areas that are the ones that give a hard time to any raytracer to clean up and waster resource.
Laso using brighter materials or that reflect more light, but this should be managed carefully because 100% whites or 100% blacks also will introduce noise in your rendering.
Now you can setup Art to render using time or samples, maybe 5 hours is not enough to clean your scene, maybe it needs more time.
Needless to say, your image looks pretty good, there is some noise but is not too dramatic. What border me more is the lack of artifical light in the first floor.
Sometimes some noise is OK, unless of course it is giving the wrong information to the viewer.
