You decide when to switch. The plant will continue to grow as long as you keep it in the 18/6 cycle The plant will double to triple the size while in flowering. so a rule of thumb is when the plant is 1/3rd the size of the grown area, its time to switch
Expect your plant to stretch more on the 3 times it's size side. Looks pretty sativa dominant. Depending on your height restrictions, or regardless in my opinion LST those branches and veg for another week or more, Longer the better. I think you need more light as well, she looks a tad stretched probably reaching for the light. Try to get some side lighting before flower, more light tends to help minimize stretching along with a good mixed spectrum. Another trick to sort of slow our flower stretch is to FIM your tops the day you flip lights to 12/12, she should recover in a week or so which is fine since the first 2 weeks of 12/12 are still sort veg weeks, it normally takes a few weeks depending on maturity to show sex. I have vegged for almost 2 months and saw pre flowers in the first week of 12/12.