Human hair grows about 1/2 inch per month. According to WebMD, hair growth and hair loss both occur randomly, without regard to seasons or cycles.
Hair's active growth phase is called anagen. Anagen-phase hairs are new. As their root cells divide and multiply, the cells extend through the follicle and push out inactive hairs that have stopped growing. It's during anagen that hair grows about 1/2 inch per month. Hair continues at this growth rate for as long as six years.
The length of an individual's anagen phase determines how long the individual's hair might grow. Difficulty in growing hair beyond a certain length can usually be attributed to a relatively short anagen phase.
Proper nutrition and massages to increase blood flow to the scalp may improve hair's resistance to breakage. This helps an individual maintain hair throughout its active growth stage so the hair reaches its full potential length.