"You can do a three character scene with HDRI lighting and background in well under 10 minutes with a reasonable class machine (NVIDIA 7 series card and up, Quad I7 processor, and at least 8 gigs of RAM).

As soon as you start adding scene geometry, the time goes up significantly since all that bouncing around of light takes time to calculate.  I can't get an indoor scene where all the furniture and walls are geometry to render in anything less than an hour with a dual quad core I7, 72 GB ram, NVIDIA 970 card. 

When the scene gets complex enough that IRAY has to fall back on CPU processing because it all doesn't fit in the RAM on the video card, things slow down significantly."

Is that a typo or do you really have 72GB of RAM? Holy s###!  What kind of machine do you have?!