It all depends on how the object is to be used. Let's say the model in question is a weapon. Is it going to be a first-person weapon? Third person? Or is it just going to be sitting on a shelf? Or maybe it'll be sitting in a glass case that the player will see from across the room. All these things are important when considering the poly-count of an object. You should take into account the following factors:
-Is the object animated?
-How close can the player get to the object?
-How large is the object?
-Can the player interact with the object?
-How much of a role does the object play in the game?
-What is your target audience?
-Is the object something that the target audience would be familiar with?
Also, keep in mind the size of texture you plan on making for the object. A 512x512 texture will look very out-of-place on a 4,000 polygon model, and a 2048x2048 texture will look like crap if it's applied to a 50-poly model (unless the model is something like a sign or billboard where the texture is the focus). You need to find a good balance between the detail you put in the texture and the detail you keep on the model.
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