Usually when the plant is close to being root-bound then the whole rootball will look like the bottom does in those pictures, mostly covered in a mass of roots.
I think it can stay in those pots a little longer. Look at the link below for pics. It might also "tell" you when it does become rootbound. The plant will droop and probably curl downwards. http://forum.grasscity.com/sick-plants-problems/1182444-help-drooping-leaves-pics.html