you can switch when ever you want really but people usually choose when to flowerr going by the size of the plant as it will atleast double in size but its common to go even more, for example if you only have 2 foot in hight i would switch to 12/12(flower) when the plant is just under 1 foot to allow for this stretch
not really aslong as you got the space etc to keep her going, some breeders have had mother plants in veg for years
keep in mind that your plant will easily double in size when you switch light cycles. the stretch can murder your grow if you don't have enough space so plan ahead!
LST can control that height spurt effectively. When you flip could be dependent on the size of your space, that certainly is a consideration, but I expect most growers flip based on their timing needs -- how long they have until they need/want to harvest.