Old friend to this site Stuart Christensen breaks down the basics for using Global Illumination in Maya using Mental Ray showing some tips to optimize render times.
Stuart Christensen writes: Global Illumination rendering with Mental Ray in Maya can yield some stunning images and impart an awesome sense of realness to a scene. There is a process to using this render technique and this tutorial by Stuart Christensen should help de-mystify Global Illumination (GI), and show you how to control the various elements to minimize render time. GI has the potential to create unusually long frame renders and you need to start small and work your way up. Remember, every surface and color effects the render so choose your textures and shaders carefully! Enjoy.