For an entire scene, it varies so much. Plenty of games modern have up to 1-11 million tris on screen at a time. What you should be optimizing for is avoiding having lots of triangles that are smaller than 2x2 pixel grid that GPUs render in.
http://blog.selfshadow.com/2012/11/12/counting-quads/
The viewmode is called quad overdraw in UE4.