www.loreal.com states that Asian hair grows at a rate of 1.3 cm per month... Caucasian hair grows at a rate of 1.2 cm a month... and African hair grows at a rate of 0.9 cm a month.
yes, you read that right... on average, we have a slower growth rate.
let's assume that 3 "average" subjects from each of the 3 races shaved their heads and retained every inch of growth in a year. the Asian would have 15.6 cm, the Caucasian 14.4 cm and the African 10.8 cm. in inches, that is 6.1 inches, 5.7 inches and 4.3 inches respectively.
if all 3 of them had the "average" anagen phase of 3 years, the Asian would be able to achieve 18.3 inches, the Caucasian 17.1 inches and the African 12.9 inches.
now obviously there are people who fall outside of "average." very few people will be at an extreme of very short hair or very long hair. most of us will fall in the middle. just to give you a visual example...
my hair is 13 inches here (layers):
(keep in mind that i am 5' 9" so for women who are the "average" height of 5' 5", 13 inches will probably look longer on them.)
as long as you are doing things to prevent your ends from breaking off or needing to be cut off (which is what we mean by
rentention), i believe most of us should be able to grow at least 12.9 inches of hair within 3-year span.