When do you switch to 12/12 in vertical setups?

Do you switch to 12/12 after x weeks or do you switch when the plants have stretched to a certain height for example 50% of the cage?

It depends on 2 things, your desired finishing height and what type of lighting you are using for veg. My strain (super lemon haze) would at least double its height during flowering stretch (first 2 weeks) using regular horizontal HPS. If you have already switched to horizontal lighting then u might get 1/2 height increase. I find most of the plants effort is horizontal stretch towards you vertical bulbs. Personally I keep to horizontal bulbs until flower, vertical bulbs seem to confuse the fuk out of my babies during veg and they do some wierd things.

Thank you very much MrMeanGreen, your answer helped me out. I have 2 x 600 watt HPS and the plants grow into very bushy, compact plants. I guess I will wait until they reach 1/2 of the vertical space.