It is subjective too as it looks. I have a animated scene that takes nearly 4 hour to render for 1 frame
For the noise i would look first at the AA, detail settings in the render node.
Then for the Quality option in the Cloud node. And at last in Atmosphere node Quality settings.
It depends on your settings like if you have Receive shadows from surface on or off too.
Write or post some images of those settings here for more help if you want.