To save time in doing 3D projects, I normally turn off G.I. when modelling and fine tuning scene objects. Then test render G.I. light settings with low res render to check if the feel of the lighting is OK, even turning the AA down. So I can check the overall feel of the image very quick. For checking details, I will set the setting to at least medium, but only render a crop section of the image I want to check, again that would be quite fast because I don't need to render the whole image. Once I am happy, then I can render a reasonable size render (1024 x 768) for final checking. If that is OK. It can go ahead in full hi quality render.