But in the real world what's a reasonable limit to put on triangle counts in a game these days?
That's like how long is a piece of string? You can't possibly generalize on how many triangles a game should consider reasonable.
Use as many as you need and no more. It sounds trite but that's what you have to do. You have to make a judgement about how close things will be to the camera, at what point does adding polys stop making it look better? At what point does reducing polys start making it look worse?