I render using LuxRender, which has native support for network rendering. So far, my longest running render took about 90 hours (master plus one slave)... (Discovering the Sword in the Stone, which has two volumetrics in it: a homogenous volume for the fog creating the “ray of light” effect on the sword and a clear volume for the water)
King to A5 (NSFW) was another long one, keeping the master plus two slaves (all fast i7 machines) running for a bit over 62 hours. Again, volumetrics were used (all the chess pieces are glass2 clear volumes).
To be clear, when I give times, I am talking wall-clock time, during which all machines are concurrently rendering. So the 62 wall-clock hours translates to around 248 CPU hours (one of the slaves has twice the cores (8) as the master and other slave (4 each), so does about double the work).
TL;DR version: Volumetrics are SLOW!
(Both linked images are also on-topic and feature Genesis figures. The one on the right in Discovering the Sword in the Stone and both figures in King to A5.)