There isn't any mention of limits within the documentation, and the parameter is a GLfloat instead of a GLclampf, http://www.opengl.org/sdk/docs/man/xhtml/glPolygonOffset.xml. There also isn't any mention of limits within the DX documentation http://msdn.microsoft.com/en-us/library/windows/desktop/cc308048(v=vs.85).aspx.