Depending on your AA sampling, this render could be done extremely quickly or, extremely slowly, I advise using RTA and RTO...
What is your AA sampling set to? If you use AA=8 at default sampling, you'll likely have a very long render time because the ray tracer is needing to take 16(minimum) samples per pixel rendered.
If you use '1/64 max samples', this will give you a very fast render time for objects/atmo using the raytracer because the 1/64 sampling levels will only take 1(minimum) sample per pixel rendered and use *up to* 64 samples in areas that require the higher AA.
If the AA is set to take a *minimum* of 1 sample(i.e. AA=8@1/64), you're best using a higher cloud quality in your clouds/atmo, a cloud quality level of '1' should be sufficient to remove grain, with sampling levels of 1 minimum per pixel.
The more pixels you use in the *minimum AA sampling - the more you can reduce the cloud and atmo quality, because the AA will be more concentrated over areas that need it, whilst still ensuring that each pixel receives, at least, one AA sample.
I try to never go below 1px for minimum AA samples, this ensures you have a clean(usually), noise free render and that areas that require it will be assigned more samples(up to 64 per pixel, if AA=8@1/64 samples is used).
It's really quite intuitive once you start to know how to manipulate the AA to work best for your scene.
