its kind of hard to tell from your photo. Generally I trans plant once the plants are as tall as the container is high and the leaves extend past the container. there is a good sense of intuition needed here as well.
It never hurt to take a peak at the root ball either. if its possible just tilt the pot a bit with your hand at the base of the stalk for support. If you end up dumping a lot of dirt as opposed to the whole thing coming out in one mass, Its too early. If the dirt and all comes out as one mass Its time.