Sampling is one of the most delicate features of Mental Ray (and most of the other renderers) rendering-time and quality-wise. Some Maya-renderer users tend to use only the default preset render settings of Maya and this technique leads to very drastic rendertime increases when dealing with Mental Ray. To properly tune rendetimes and image quality to your taste you have to understand a few fundamental concepts and techniques.
http://www.impresszio.hu/szabolcs/MentalRay/MentalRaySamplin g.htm
http://www.lamrug.org/