Beards can reach full length in as little as two months or as long as six years. The final length of a beard is determined by the length of the hair's growing period, which is called the anagen phase. Everyone's anagen phase is different, which is why there is such variance when it comes to the time it takes to grow a full beard.
The speed at which beard hair grows, as well as the length of its anagen phase, is determined by genetics. On average, facial hair grows at a speed of one-half inch per month. Therefore, for an average adult male, growing an appreciable beard should normally take a couple of months, though this is subjective.