hi there, welcome
I'd say alot depends on what you're modeling, what kind of game you're modeling for, and how important the object is in your game.
If you're modeling a whole house then that'll use more polies than the garbage can that sits next to it, obviously. FPS games will have more detailed props and buildings, while rts games can do with lower polycounts compared to fps games. (say compare a building from a rts to a building from a fps). Same with the importance. If you're modeling the back of the tank, that you character has to run behind for cover half of the game, then you might want to put some extra detail into that, while the garbage props can do with insane low poly counts...
Rick Stirling wrote two articles on polycount too
rsart - Rick Stirling, games artist » Blog Archive » How many polygons in a piece of string?
rsart - Rick Stirling, games artist » Blog Archive » Yes, but how many polygons?
cheers