Expected growth when flowering?

I read somewhere on how much growth to expect when switching to 12/12. But I forget how much it said. If plants are a foot tall then they could put on another 2 1/2 feet when complete?
I'm working with a height of 5 1/2 feet from the bulb to the floor using a 1000w. If the plants are in there vegative state and at 15 inches already, should I have flowered sooner? And since I didn't flower sooner should I be pruning the plants now to control there growth?
How do I determine the proper time to flower by my grow dimensions? I can put my hand 16 inches from the light and it doesn't feel too hot.

depends on the strain u are growing? indica double in size the first 3 weeks and hybrids and sativas will stretch 3 - 4 times the size.....

I'd look into BushMaster from Humboldlt County. Stops plants vertical growth, promotes branching, decreases flower-time by up to two weeks.
With a 5.5 ceiling, and the heat of 1000HPS, it'd surely help if you could grow small and bushy.
I use it . . . works as advertised. Around 40$ for 8oz's, which'll last me thru death and the burial.
BushMaster Information Thread - The Garden's Cure

Thanks for tip, I ran out today and picked some up! Along with a fresh light to flower.

top the plants?

I have topped a couple of the plants others Im leaving alone. Im going to use the bushmaster and see what happens.

well than good luck to you, and get some pic of these babies up.