It depends 100% on the hardware you are targeting. The best way to answer this question is to choose a particular target graphics card and PC configuration, and get an example of that combination in front of you. Then see how many polygons you can draw before your frame rate sucks.
Note that you’re almost certain to hit the Geom limit before you hit the polygon limit–modern graphics cards have a ridiculously high polygon limit, but pretty much the same Geom limit they have always had.
Make some dummy model, for example use makehuman model.
And just load it in scene many times.
I think that mh model has about 15 000 quads, and if i am right it is recommended to keep geom count below 300.