Similarly, you may ask, how long does mycelium take to fully colonize?
After two weeks to a month (depending on temperature and substrate size) the substrate will be fully colonized and it's time to change environment. During incubation the mycelium should be kept in a dark and warm place.
Correspondingly, how do I know when mycelium is fully colonized?
There's no guaranteed way to know, but that's a good rule of thumb. If you take it out of the jar before the inside is fully colonized, it will sit in your FC doing nothing until it is anyway and open to contam for that much longer.
Light. A commonly held belief among growers is that mycelium will grow faster in total darkness. There is no data to support this premise; however, significant exposure to direct UV light from the sun can be detrimental.