The plant will stretch, and may stress. It wont flower straight away, it'll veg on 12/12 untill its mature enough to flower, and then it'll start to preflower, and then go on to flower. You'll get popcorn buds, small yeild and a stretched plant... and maybe hermies too.

When the plant is mature enough to flower, it'll measure the un-interuppted dark cycle which will release a flowering hormone.

It's best to grow a plant properly.

Also, judging by your name, if our growing LR's you'll be wasting your time. As you probably knows, they are auto flowering, if you put them into 12/12 you'll just decrease the yeild dramaticly, the lowest you want to go is 16/ 8