When 12/12ing

I just wondered that when you first put your plants into flower should you leave them off for 36hours then 12on 12 off daily to help them go into flower also what if I put clones on 12/12 straight after they have rooted til harvest thanks

You should only flip your rooted clones to 12/12 right away if you're doing a sog type grow. They really won't yield much, especially if you only have a few of them.

I've got 20 to put in 2 square metres for sog

20 for a 2 sq meter sog? To each his own I guess. But just be sure the clones have rooted well, not just one or two roots sticking out and you're golden.