@MrGrowwww Hi mate if running autoflowers you can run from seed to harvest on any light schedule be it 12/12
18/6 20/4 or even 24 hours a day although I think the plants need atleast a few hours sleep so I am doing a 21 on and 3 off schedule
Now when you are growing photoperiod strains most growers will veg with an 18/6 cycle before flipping to 12/12 to induce flowering (photos can I only flower if given atleast 12 hrs of total darkness ) Hope this answers your question and happy growing