I have not an answer that is correct, but I doubt it even exists.
I can tell you what I do (with Unity).
For important objects, around 10.000 – 15.000 (max 20.000) tris ( example: tower, helicopter, tank, everything that moves, comprehensive of all possible variants).
For environment (of course, modular), 200 – 600 (max 1.000) tris.
For character: TBD, I haven’t done that yet.
I’m doing this for a PC/Console build, and you should really take the target platform into account when deciding the “quality” of your work. For phones you want few triangles, smaller textures and less code (if possible).