Not too sure what the scene was that you posted up, its referencing files so I only get the light set up.
The render time all depends on what your settings are, are you using a high Anti aliasing setting? Raytraced shadows, Mental ray with all the bells and whistles on, how heavys the geo etc etc? So 20 mins (depending on your machine and settings) might be about right, make sure if your comp has multi cores that use all avialable porcessors is on.
Theres a neat little trick you can pull to make maya render without being loaded up (but alas i'm not too sure it works on a mac but might be worth a go) all you do is right click your file and select render, off it goes rendering, this speeds up a render no end.
You can also render form the command line, and from a text file but again I only know how to do this in windows, and again this renders without maya being open.
Theres also a few other tricks etc, but as I'm not too sure if its something easy to fix in your scene its a bit hard to suggest something.
"No pressure, no diamonds" Thomas Carlyle