How long should i wait??

hi, i was wondering if anyone could help me out with sum info, i hav some seedlings growing away , they are bout a week old and doing great, now wot i wos wondering was how long do i keep the seedling growing before i introduce them to my flood and drain system, currently the now they are in 4" rockwool blocks and get watered nearly everyday with a young plant feed solution. i have all my flood and drain system ready to go, just need to know when to put them in ??????? any help would be much appreciated. Thanks. :cheers2:

Ps those pictures are from 2 days old.