You can switch it to 12/12 anytime you want but it won't instantly start flowering. It will need to be mature enough, which is what you are asking I think
. How long that is depends on the genetics and environment, and isn't something anyone can tell you. I recently did exactly this with a seedling. The seedling was about 3 weeks from germination and had about 4 nodes, so still pretty small. It only took about 7 days before I saw the very very first signs of sex(I was watching very closely). It was definitely not flowering yet FYI even after 7 days under 12/12. Once I saw it was a girl, I pulled it from flower and it went straight back into 18/6 light and I topped it for a clone. It is happilly vegging again and is working on having 4 nice main stems. The clone looks great too.