Below is an image rendered in Mental Ray in Maya 2013.
What is the difference between the football on the left and the one on the right?
8 hours of Render time.
The only thing different between them? Materials. They both have the same exact material settings and the same exact texture hookups. The only difference is the football that took 8 hours to render had materials that were created in Maya 2011. The football that just took a couple minutes to render had materials created in Maya 2013.
Apparently Mental Ray does not like to render things created in previous versions. Best advice-recreate EVERYTHING in the version of Maya/MR you are using. Helps debug issues like these more quickly. #lame