Reck is right, GI SD takes 3 times longer to render most of the times.
In the least worse case scenario it takes twice as long.
Also, as Oshyan pointed out, it's likely due to the ray trace atmosphere in combination with high atmosphere samples.
How many atmosphere samples do you use now?
I don't see clouds, but if you still use them somehow, how many samples do they have and which quality setting does that achieve?
Besides that the seemingly small increase from 0.7 render detail to 0.9 makes a huge difference, actually.
Not only will the geometry be subdivided much more (likely beyond the sub-pixel level), also the GI calculations are linked to them, exacerbating the rendertime indirectly.
Also, you probably use a very low gradient patch size in your compute terrain to get those walls look good, so that also may contribute to the longer rendertime, especially if you render at such high detail levels.
Looking at the image and your machine specs this should render under 12 hours easily I'd guess.
However, I wouldn't be surprised this also takes so long because of the huge amount of geometry you've imported with all those ivy's. Damn!
I'd restart the render, sorry, then use the following for a crop of the area with the most contrast, so darkest shadow vs direct lighting:
Detail 0.75
AA6 (full)
GI 2/4/6
No GI SD
Atmosphere samples = 32 (NO ray trace atmosphere)
Keep soft shadows as is (default is 0.5 degrees which is the soft shadow radius like in real life, so 1 is a bit exaggerated effect, but can often be desired)
This should render muuuuch faster.
If you're not happy with the detail of the ivy's in the shadow you can increase the GI from 2/4/6 to 3/6/6.
Good luck!
Cheers,
Martin