My experience so far has been you can expect to be able to render about 300 to 500 images (Flash) or 1,500 images (Desktop) before performance becomes an issue. A top-down racing game could be easy to make within those limits. A platformer, on the other hand, could easily push the limits depending on how many tiles you have and how deep they go.
For real 3D, those limits are probably lower. My numbers came from isometric projection, which has simpler math.
It'd be interesting to see how far into the world of 3D Stencyl can be pushed.