I start flowering usually a month into the grow, no more more than 6 weeks, as I usually run on a 3 month grow cycle.
Plants are looking ok so far, as for the growth, you can at least expect double what it is now in terms of mass (If it's 1 foot now it will probably go over 4 foot) If you haven't cut the top of them.
If you have cut it, your plant will grow outwards instead of upwards so there's no clear cut answer.
I find 1 plant needs 2/3 foot width with a 4-6 foot height.
The longer you leave it to veg the more potential flowers it will grow due to more space being on the plant.
Now you've started flowering DON'T CHANGE BACK, you will stress your plants out and I don't think it's worth it to do that now to gain an extra 2 weeks of vegging.