There is no solid guidelines. It mostly depends on if you want to make something for yourself, or something for everyone. Just some numbers, the F-15C is 160,000 triangles, the AIM-120C has 2,500 triangles, the Predeator drone has 8,300 triangles, the m-1 has 21,000 triangles.
You can open existing models in the model viewer. In the top left it will show you the triangle count