How long to regrow hair?
According to https://en.wikipedia.org/wiki/Human_hair_growth:

Scalp hair is known to grow between 0.6cm and 3.36cm per month. The growth rate of scalp hair somewhat depends on age (hair tends to grow more slowly with age), sex, and ethnicity. It was previously thought that Caucasian hair grew more quickly than Asian hair and that the growth rate of women's hair was faster than that of men.
However, more recent research has shown that the growth rate of hair in men and women does not significantly differ and that the hair of Chinese people grew more quickly than the hair of French Caucasians and West and Central Africans.

Depending on your age, follicle-condition, nutrition or possible inhibiting factors it can vary how long it will take until your hair has grown back to the previous length.