Plants in rockwool slabs until sexing?

I have 3"x3"x4" slabs... planning on leaving them in just rockwooll without hydroton until i switch to flowering and can eliminate the males.

how long do you guys figure i can leave them in the rockwool before they become to rootbound and need to be transplanted into hydrton?

thanks

That's really hard to say as it depends on a myriad of factors; however, that size rockwool would be fine for sexing plants. Even if they become rootbound, they will still show sex (they'll just stop growing in size).

It won't take long for the roots to poke out of a slab that size.