When do switch to 12/12

Hey, I just planted a few seeds and they have began to sprout I was just wondering when it is best to switch the light from all the time to 12/12?

not yet m8 ...wait untill there growing qite well at least 5 inches... and have alternating nodes

Whats the minimum amount of nodes, before u can flower?

there is no minimum amount of nodes, but know this, the bigger and healthier the plant is before you force flowering, the bigger your yeild will be in the end.. i like to wait till i see one of two things, or maybe even both if possible.. one is like the above poster said.. as the plant matures it will stop putting out even nodes and will start to put them out unevenly.. i like to wait for this or to see signs of preflowers.. usually they happen around the same time.. or about a week or so apart in most cases..
forcing a plant to flower earlier than it is ready is only going to reduce yeild and quality of the smoke in the end of the day...