depends on how much room you got say you got like 4 foot of open room in your area so you dont want it any bigger then 1.5 feet they grow pretty fast when flowering
you don't want your plants getting any bigger than 12" in veg. when you flip to flower, there will be some stretching. if your plants are already pretty tall, they'll get even bigger during the stretch. they can sometimes double/triple in size in flower.
that's what you decide. you go by size, not time. when they get to the right size for your space, you flip em!
The plant will take however long you veg PLUS 2 months of flowering time. The 2 months they talk about has nothing to do with veg time.
This also varies, if its seeds then you get more stretch than with clones but the rule is the same flip em according to your space requirements. You can always train if the genetics make it go crazy. Generally i veg seeds for 1 month pruning them into little bushes but i work with clone only strains so theyll veg the whole time side by side with the flowering plants(8-9wks) and ill cut more then throw em into flowering.