Hey!
To figure that out you have to measure your space.
If you grow tent is 150cm tall.
You need about 30(11in)-40cm(15in) of distance from the light, take away the height of the pots.(let's say 20cm(8in)).
You left with about 90cm(35in) of grow space. That means she can grow until about 90cm(35in) safely and not get light burn or stress.
Knowing that.. You need to know that in the switch to flower they can stretch and double even triple in size depending on the strain.
Some go upwards, others outwards and some do both.
So to play it safe.. When they're at about 35(13in)-40(15in)cm you can switch to flower, that way they can stretch till 70(28in)-85(34in)cm and you have a little more to play with..
Hope this helped.
Good luck and happy growing.