How about testing it? Because there are so many individual parameters it is hard to tell. Especially because things depend as well how the polygons are organized. Not only how many. Try terrains and you can handle a lot more than imported trees from XFrog or TreePro. So i think you simply try it out by yourself and let us know. My biggest so far was around 60 million polygons, but it was not my slowest. I heard from others that they even have dealt with much higher counts. Again, not necessarily the slowest files. The slowest i had had only around 1 million polygons and was only an imported conifer from TreePro. I gave up on that.

One day your ship comes in - but you're at the airport.