When do I introduce light?Updated a year ago

The answer depends on which stage the bag is currently in. After inoculation, you may leave the bag in the dark. We recommend the bag to be placed above waist level in an area with low traffic but available fresh air. 

Light is definitely beneficial. While it's not a necessity, your final product will grow a lot better if you do have it. Indirect sunlight is sufficient, when your substrate is fully colonized. If you choose artificial light you should have a timer for 12 hours on and 12 hours off.