I don't know - the creature sculpt I'm working on is still nice and smooth in both sculpting and moving around in the viewport, and Blender says it's on 44 million quads.
@MrNinjutsu : Well, it just works. You can lay down decals at the highpoly stage (thus avoiding the unnecessary modeling of any small details) ; but it is also possible to add such details after a game asset has already been completed. In both cases you end up with two sources : a regular high (or a previously athored game…