By my experiences, very important are materials. I had one scene with mirror walls and a lot of high reflective materials. I tried ultra settings and estimated time was 6 days for 1024768 render !!! Also, if you have volumetric atmosphere, a lot of lights, glass materials etc., everything dramatically increases render time. BTW, last week I made my biggest render (Still life image from my gallery) 124009400 pixels at 300 DPI :) It took 16 hours in final mode on Athlon 1800XP and 256 MB RAM.