how long till i can put seedlings under hps

right now i have 2 seedlings started 12/18
i started them under a florescent tube but picked up some cfls today to get them started
i have them in a homemade areo system
I was wondering how long i should expect to wait till they can handle a 400 watt hps light?

thank you for looking!
Joe

12/18????????????????????????????

wait till they have roots and then you can put them under HPS if ya want to.