i grow organic. i only feed water from start to finish, so i can't really help with the the nutes question. generally from seed, it's best not to flip the lights until the plants have become sexually mature. alternating nodes, and/or pref flowers are the best ways to tell if your plants are mature.
re-veg times vary. it has a lot to do with your experience as a grower, as well as your experience with the strain. there's subtle changes that happen as the seedlings grow into maturity. predicting when they are on the cusp, and acting on it will allow you to determine sex with little to no re-veg time.
during my first run of southern lights at 5 weeks from seed. when watering i was observing the new growth. what i saw were nodes sprouting in pairs still, but offset about an inch. so i took a close look at the small very new growth, and could see that the nodes would be fully alternating by next week. this gave me the impression that they were on the cusp of adulthood, but still no pre-flowers. later that day i got some really good advise from 4u. he suggested that i put my plant's in darkness for 48 hrs. so i thought about it, and it actually makes a lot of sense. the amount of darkness is what drives flowering. when the lights go out, the plants stop producing antiflorigen. after 8-12 hours of darkness the anti florigen has be suppressed enough that when he lights go back on the next day, the increased levels of florigen will be able to react with the phytochrome to produce flowers. maybe immature plants need extra time to lower their antiflorigen levels. so giving them 48 hours of darkness gives them enough time to decrease the levels enough to induce flower production to begin.
many many bongs rips then started to ramble...hope this helps