There is no set value for this because every object is different. You might use over 100,000 polys on a bus, but using 100,000 on a fence object used lots of time on tiles would be a bad idea.
One of the best ways to get the best performance is to ask "are these polys needed? Will this look noticeably worse with fewer polys?". If you were making a house for example, you wouldn't need polygons on the bottom because they will never ever be seen. My sign poles are octagonal with smoothed edges so you can't see the joins and a gradient texture to help give more illusion of roundness. Indeed if a round thing is small enough on screen it could have a square or triangular cross-section or even be 2D. By only using polygons where they're required, polys and therefore processing power are not wasted. If that still means low performance, so be it. The only way then to improve performance would be to degrade appearances.