There are two things you want to look at before deciding when to flower, (a) is the plant sexually mature enough to flower?
There are 2 features that distinguish mature veggie plants from immature ones. They are alternating nodal arrangement and leaves with 5 or more blades. You can flower before those features are present, but the plants perform a whole lot better if you wait. Most strains except for equ. sats have these by 12" or so. Personally, mine have always had these features before they reach 30 days old. With CFL's, it will take longer to veg.
(B) is your lighting system, theres no need to veg until your plant is 18" tall, if all you got is a 75 watt fluorescent light. Is that a T-12, T-8 or what? If its a basic shop light, its probaly a T12, not good for flowering.
What will you flower with? What is the penetration of the lighting setup that you'll choose to flower with?
My point is, theres no point in growing a 36" plant with a light that only penetrates 6" .
I hope this helps you.
Peace