Hi Nizmo, what are you growing? Is it an aotoflower? If it is an autoflower you don't need to cover it, it will flower regardless of dark period but would benefit from additional light. Most auto growers do something like 20/4 or 18/6.
If it is an autoflower and as not shown any preflowers by day 40 then treat it as photo, see below
If it is a photo what is important is dark period, I would cover I so it gets at lest 12 hours of dark. Also if is a photo you should post question like tis in the photo section. Here is the link