The longer you veg the shorter flower times. Reason is that the plant has a natural maturation process that flows from vegging to flowering to maturing. Our manipulations with the lights are to speed up the natural process.
I usually veg about 5 weeks or until the plants are 20-24 inches. With that short veg time I can pretty much count on my plants doubling in size while flowering. I'm set up so my plants finish about a foot under the 1000W bulb. If I vegged an extra 2 weeks I could knock a few days needed for flowering.
Yes, I can and do top and FIM my veggers any time in the vegetative cycle. Bonus -- a good way to keep 'em short is to make a clone out of the top cola. Topping will keep the height down as well as make the plant bushier. Bonus is that clones from topping do extremely well in the clone domes.
Good luck, BigSteve.