Ok, I've found some settings which together add up:
1) You're using Ray Traced Atmosphere (RTA) with AA6.
For a ray traced atmosphere that's a rather high AA setting, so consequently you can drop the quality settings for your clouds from 1.02.. to roughly 0.3.
64 atmosphere samples is already quite high for non-RTA renders, so with RTA AA6 you can definitely render with 8 or 12 atmo samples, no problem. (8 is the bottom limit).
2) GI is set to 2/4/6, which is good when rendering vegetation and complex lighting, but I think your image does not require this specific criteria. GI 1/3/8 should be fine.
If you want a bit stronger GI effect you can also increase the "strength on surfaces" setting in the enviro light node from 1 to 1.5 for example.
3) Last is the soft-shadows...I don't see regions where it's absolutely necessary to have these.
4) Not something you can really change now, but the more multiplying/adding/mixing and blue nodes you have for defining cloud-fractals the slower they are.
So in this case I'd leave them untouched, but perhaps something to keep in the back of your mind for the future.
I think if you adjust the first 3 settings you'll chop off quite some rendertime. I think/bet it will render <10 hours then.
Cheers,
Martin
