Unless you have enabled the options below this shouldn't render so slow.
1) your sampling is customised. How? full sampling isn't really needed here, as it only works for ultra-fine jagged details, which aren't present in this case.
I'd go for a sampling of 1/16th first sampling.
2) detail blending? (Extra tab) Although set to 0 by default, maybe you increased it back to 1 for some reason? If so, it would at least double the rendertime.
3) check atmosphere/cloud raytracing settings. These settings are only required if the terrain casts shadows into the atmosphere or onto clouds, which is not the case here. So disable raytracing for the atmosphere and/or clouds if you had them enabled.
4) check quality level of clouds. A detail level of 1 should give smooth noise-free clouds, unless you have very high density/cloud depth settings. Then you might have to go to 1.5 for example. I've seen tgd-files of people here which had cloud quality set at >20, ridiculously high.
5) check sample level of atmosphere. For this scene I'd estimate that around 80 samples should give nice results. I don't know how many cloud layers you have? The more, the higher the atmosphere samples I'd advise. However, I'd never go much higher than 128 samples in scenes like these.
Further some comments on earlier suggestions:
GI blur radius has no effect on rendertime.
PorcupineFloyd's suggestions are good. One little remark is that a detail level of 0.8 would result in GI detail of 0.8 as well which is low.
(GI relative detail = 1 and detail = 0.8 --> GI detail = 1 x 0.8 = 0.8 --> the "relative" means it's relative to the detail setting)
Clouds really benefit from good GI settings.
So by definition I'd never disable GI when rendering clouds, without it they look drastically worse.
If none of these things improve it then I'd strongly suggest to either post or IM the tgd-file.
Cheers,
Martin