Newgrow, I can't believe you are asking these questions when you are an experienced grower. I saw your thread about your Lowryder grow (that was you wasn't it? or was that Heartsurgerysurvivor?
Anyway, I seem to remember seeing your stuff no?
Assuming you're going into hydro after enough germination in Rockwool, I would wait until you see just the tip of the taproot coming out of the bottom of the cube (1" cube right?), and then plant it into your hydro unit. As long as the temp is not too high under your 400W, the sooner the better. Fluoros will do fine for the first week, but get her under your good light right away. I have grapefruit going right now and they were ready in 6 days from seed to go Hydro. I have used Rockwool before, this time I used those grow plug things by GH. They are made from tree bark or something. They are pH ready, and need no added nutrients for the first week. But Rockwool is great too, as long as you know how to pH adjust and use light nutes to the right degree at first. I've always had success with it, but this grow I tried something new. I used the 1" cubes and planted to clay pebbles (Hydroton) in an ebb and flow.
I consider seedlings as the vegetation state too. As long as you have good conditions or better, everythiing is "dialed in" right, you should be ready to flower at 30 days from seed for a plant with good genetics. The longer you veg, the larger your plant may be, but that can pose a problem to the indoor grower. Some people report of flowering at 14 days from seed, to keep their plants short, resulting in less yield, but tailored to the space they have to work with.
Here's a quick checklist for you. Abide by the "laws" of good farming and you will achieve success, always.

Genetics (seed type)
Proper light and dark
Correct nutrient regimen
Constant pH monitoring & adjusting
Proper ventilation & temperature control
Adequate height and spacing (room to grow)

It is best to grow fewer plants with plenty of room for "full potential"
then to crowd as many as you think it will hold, in hopes for a higher yield.
in certain strains like AK-47, side branches can grow colas as large as the top of the plant so leave enough room for them to get good light. I will post a pic of 6 crowded AK47 that should have been only three from the start. It's hard to chop them down, or give them away when they are so healthy although crowded.
the last pics are the latest grow. 2 Grapefruit and 1 Easy Bud (Female Seeds .nl) I got from Dr. Chronics. The GF are large enough for what I have room for but I really want to see how this EB turns out. It is shorter than the GF but it very stout and thick. It is also indica/sativa cross which is different then the pure sativa of the GF.
I'm at 44 days from seed, 15 days into flowering. I hope to harvest them around September 21st. I may bend 'em down in my first attempt at screen of green (sea of green), and maximize the canopy space and light to the plants. I'm interest in seeing just how tall they will get though without being tied or bent down. I had a problem with lack of height in my last grow. Probably because I vegged my AK47 for 35 days (5 weeks). A bit long for the height of my grow space.