Hey Noora,
Theres nothing wrong with veg'ing them for as long as you want as long as you take some precautions.
First of, you must remember that they will just over double in size once they are flowered so make sure you grow space can accommodate.
Second, what size pot are they in? Depending on how big they are and how big they are projected to be at harvest, it may be smart to repot before going 12/12. A good rule of thumb is about a galon for every foot of plant. So if your girls are about 2.5ft right now, put them in a 5gal so they are comfortable for harvest and dont get root bound.
Third, if space does become an issue remember that you can always LowStressTrain your plants or you could consider some pruning. You want to do most of your pruning before you start flowering or in the early stages of flowering.
I hope that helps, feel free to ask any more questions, I'm glad to share what I have learned.