when you see that little loop appearing from the top of the rockwool just before it pops upwards you should get your light on it and remove your humidity dome (if using one).it will depend on how deep you put the seed in the rockwool before it pops up or IF it will pop as i reckon your always better germinating the seeds before planting as you can control the temps and moisture better and know which seeds actually pop open.I take it the rockwool was soaked in a Ph adjusted soloution for 24-36 hours before planting?
good luck with them