I have to admit that my system is old and in need of an upgrade, although I have a 64bit system available but greatly under used.  In general, I believe scene content and complexity impacts on render time, and I try to evaluate every object for optimization and streamlining.  If you have background content which are ladened with hires textures, displacement maps, and hi poly counts, expect the renders to run longer.  By how much?  Depends on the system.