rule of thumb...you must veg seedlings at least a month.
2 is best.
If space is an issue, then you flower when plant get's to be 1/3 the height of the available grow space

I have done experiments with all phases of vegging times and ...see above.

If you have clones then you can veg as little as 2 weeks.
Now since clones take the age of the mother...they can be flowered at 1 week, but understand that is 1 week of veg after 2 weeks of rooting; And, they won't yield diddly. take the extra time and do it right.

If you rush or shortcut any stage...you will not be happy in the end. good luck