That's a lot of growing and stretching for 36 days?...anyways, you can start flowering ASAP with the 1000 HPS....next time, switch from fluroescents to HPS after the plants are about 10 inches tall....
Don't get the light too close at first...it's gonna be a big shock going from fluoros to HID lights.

I assume you have seen the sex pre-flowers? Those leaves look they have a LOT of Sativa strain in them. Usually White Widows are 60% Indica/40%Sativa.

Good Luck...enjoy your grow...:thumbsup:

dutch