Transplant once roots are visibly pushing through the rooting medium and the clone is showing fresh new leaf growth on top. For most cuttings in rockwool, peat, or rapid rooters, that happens 10 to 14 days after cutting.
You want white, healthy roots poking out of the sides or bottom of the cube. The cutting should look upright and vigorous, not limp or pale. If the clone has been rooted for more than a week and is already rootbound in its cube — roots circling the bottom, leaves yellowing slightly — you're already late. Move it.
Transplant into a small container first, not directly into a final pot. A 4 inch pot or 16 ounce solo cup lets the root system develop without waterlogging the medium. Going straight from a rockwool cube into a 3 or 5 gallon pot is how you drown young roots — there's way more water than the small root mass can pull, and it sits cold and stagnant.
Move up to a 1 gallon once roots fill the small pot. Water lightly the day before transplant, handle the root ball gently, and set the clone back under its normal light and environment right after. Treat the transplant like a rest day — don't feed hard for 48 hours, and ease it back in the way you would a newly arrived clone.