whenever you want mate but do it too early, say 3 weeks veg from seed and your plants wont be mature enough to start to flower, going to 12/12 at this point has little use imo unless you need flowering to start as soon as possible then as soon as the plant can flower it will but you may slow veg by doing this because you are giving the plant less light.
At around 5-6 weeks you plants may start to show pre-flowers (google it) at this point plants are ready to flower and sex can be determined so its ideal to wait until this point to swich to 12/12.
The plants can be left in veg for as long as you want, you could veg them for 6 months and grow some monster trees.
If you are growing from clone you can swich early too but its ideal to let roots develop well before hand, a clone is a cutting from a mature plant and can flower at any point under 12/12.
Hope this helps mate