Hmm. I don't have an exact number, but, I have had entities that are not characters that have up to 15000 polys in them. Most characters are coming in around 12000. I think the limit is higher.
But you could use more polygons for better detail or more complex models, but also keeping them down to a reasonable size is also a good idea. No sense having a ceiling light that is 300 feet away having 5000 polys when 100 would do, because the player will never be that close to the entity to see the detail anyway.
A great texture and normal map will take the place of a lot of polys. Only things that really need more poly are round things so that the profile of the object does not look like a hexagon instead of a circular pillar. wheels, pipe, fancy stuff.
Intel(R) Core(TM)2 Quad CPU Q6600 @ 2.40GHz, 2400 Mhz, 4 Core(s), 4 Logical Processor(s), 4gb RAM, Nvidia gtx660, Windows 7 Pro 64bit
