You transplant when your system will get them wet during pump on times. Don't know what the cubes are going into, but you could do it now, since rockwool is so easy to keep wet. In fact, too easy. Otherwise, you need to keep at least part of the roots wet at least some of the time. However that may be, doesn't really matter.

I wait until most roots are about 3 inches long then put them directly into hydroton in net pots. I'm looking for the roots extending to the bottom of the pots so they will get wet.