Happy grower, I have the exact same NFT system and for me, I put them in with a 1/4 nutrient solution when I had way less roots (my tap roots had just gotten through). So far I have filled my system with roots, they are into the reservoir and everything, so I'd say from my first grow experience thats more than enough roots, but your meant to initially use formulex / some hydro solution designed specifically for young plants / seedlings, I got away with it but I'll be doing that next time.
If you have any questions feel free to pm me, I'm not vastly experienced but so far the right questions asked here and a few other growers I'm doing better than I expected for my 1st grow.
All the best luck to you matey!
PS: which light system are you using / whats your setup?