You can flower whenever your heart desires man, as long as you have a photoperiod seed. Autoflowers you have no control, they're just going to do their thing.
You can veg for a week, you can veg for 6 months, totally up to you. At some point the plant is going to hit its max height because of its genetics and slow way the hell down, but you can keep it going.
Your basic idea when you choose to flower is your space, not how long its vegging. Once flowering begins your plant won't grow any new tips, no new water leaves, no new growth. What it will do is expand to make room for buds. Every internode you have is going to lengthen, the tighter the internodes, the more its going to stretch. This is why stretching is so bad, you'll have buds few and far between with plenty of stem showing. If your internodes are nice and tight, your plant looks good and bushy, expect your plant's size to double. Mk here is going to still have bulbs in the top of his box, his internodes look good, and his plants are nearing the top of his 1st tote. Hence the "flower any time", if he wants to get them a bit bigger for more yield, its up to him. I'd say his plants are big enough now to get good production out of his box. Wasted space is less weed, and he's got the light to get as much plant as the box will hold.