Rule of thumb to go by when it comes to your light and its distance from the plants is to check the temperature with your hand. If it's hot on your hand, it's definitely hot on the plant. As for vegetation light times, I've always used 18 hrs of light to do so, but I'm not saying I'm an expert, either. I just remember a site saying that darkness allows the plant to develop roots and roots are important. At about a month or to your desired height, you can switch to flower power 12/12 lighting. I hope this helps.