So I did some googling because I was also wondering about this, and it seems that you can just leave them on till its dark, but you have to be vigilant about doing it EVERY day until the actualy light period is near 12/12 otherwise plants may re-veg or hermie on you. As for how dark you need to get it, seems like judging from what I read, you don't need perfect dark, and even plentiful evening shade can speed things along. And yeah you would be better off with something that does not touch the plant.