How can you tell?

How can you tell before starting a rendering that the forementioned rendering is going to take hours to complete??

I do renderings quite often.  One might take 10 minutes and another may take 2 hours.  Sometimes there may be very little difference in the two assemblies being rendered.  What is the best method for making an educated guess as to how long a rendering might take before starting the rendering, and then 3 hours later it is too late to abort?

I also thought that this was supposed to be getting better with SW 2012

Tech Specs:

HPZ400 Workstation

SW 2012 SP 1.0

Win 7 Pro 64 Bit

Intel Xeon QuadCore 3.33 GHz

12 Gig Ram

NVidia Quadro FX 1800

SolidworksPhotoview 360