12/12 from seed. What to expect and do?

Im at 4 weeks flowering in my first grow and I got some tasty green that had seeds so I thought i'd grow one. I'ts sprouted now and I was just wondering if anyone could tell me when i should start feeding and what I feed it, I have biogrow, biobloom and topmax. I guess the topmax will go in as normal nearer the end but should I give it grow and bloom from the first feed in about 2 weeks? Also how long does usually take from sprout to harvest roughly? I'm growing in soil under 400W HPS.