I would say the limit is reached when your game slows down beyond what you can tolerate on the target system.
You really have to balance this out in order of importance.
I will say it's better to start lower, then build up from there.
You are limited by the shapes you're making too.
For a character model, I'd seriously try to keep it under 800 faces if possible, at least to start with because it's way easier to add detail than to take it away, and most detail stuff can be added with clever texturing.
You can also make three or four models, with higher and higher levels of detail to switch between depeding on the distance from the camera.
But in the end, it's a balance between every element of your program that gives you the speed, not just the poly's, so be prepared for some varied opinions on this
BTW, the question itself is flawed, are your spaceships rounded, or boxy?
for instance, if I want to make a cube shaped thing, well I'm stuck with at least 6 poly's, unless I use a sprite. Or, if i'll never see the back face in a game, I can remove that and have 5. So you can only reduce to a certain point, based on the overall shape of the object.
Personally, I go as far as possible, without losing the impression I'm trying to give
ZX Spectrum 48k Issue 3, Radio shack Tape drive, Rank arena 12" T.V. set.
