I have a work in progress scene with an HDR panorama, high quality IDL settings and a lot of polygons. The room has 83731 and the two world spheres 3200 each (I am using two spheres because I want to test if I get better IDL by using one sphere to show the panorama image and the other one for IDL lighting; so its mainly a test of an IDL proxy concept).

As you can see firefly is already asking for far over 2 GB RAM. But there is no figure in the scene, no textures for the room and no furniture. So it could be that the final scene already needs more than 3 GB.

Tested in PPro 2014 (64 Bit).