Nice sativas. A "real" sativa can take anywheres from 16 - 20 weeks to finish. So I would let her go until she's done. She will let you know when the trichomes start to turn from all cloudy to amber. Thats when to harvest! A 600w HPS lamp for 1 plant is great lumen intensity and all you would need. In fact, its a little overkill if the plant was only vegged for a month. I fit 16 plants under 1 600w HPS lamp. Vegged from seed for 6 weeks then flipped. From clone I can put 24 pots under each 600w HPS, vegg for 2 weeks, then flip.
Looks like the next round is gonna be some LST action. Nice. How long has that sativa been in flower?