There is no set time you have to veg a plant. I have a plant that I started with 12/12 light and it is a fine plant just small. Just veg as long as your room can handle. Your plants will double or triple their size in flowering so you have to be prepared for a stretch. Good luck
I totally agree, my 12/12 from seed plant is half the size of plants the same age that were vegged. You can check out my journal for pics to show the difference between the two
As said you can start with a 12/12 cycle as that is how they grow naturally or around that. Just because your on 12/12 though the plant would still be in a vegetative state as its now focusing energy on buds but primarily foliage.