Well,for me i like to over do things abit and my renders ALWAYS start as infinite render times,but never take more than an hour or 2 at most..Vue likes to "guesstimate" As for your pic being so grainy, you can go into custom settings and set your texture filtering a bit higher,but for atmos you can go int to you sky/fog/haze tab and better the quality there at the bottom to get rid of the grainy clouds,you dont want to really set both though as it  bogs Vue down hardcore.I suggest rendering in broadcast with your sky/fog haze tab up and your texture filtering set to no more than like 45-50% just personal preferance of course because if it is set to high it looks very smooth,almost unreal,but if you want that look(almost painted) then to each there own(;

"Many are willing to suffer for their art. Few are willing to learn to draw."