H0w l0ng d0 i wait untill i change my light to a 400watt

Right now i kinow its begiining stage..they are ak-48 seeds 4rm nirvanna they sprouted in my fox farm soil fast {sorry no pics]they are both 6-7inches now and just with two small leaves...how long shall i wait until i change the light ???

what light are they now?
as for the new light, 400 what? hps, mh?
either way, hps or mh are gonna give a much better result, id change rite away.