I think it took Kristin at least 18 months before her periods started, but like I said in an earlier posting, it depends on where your child was in her pubertal development to start with.  How tall is your daughter?  Kristin ended up  5’ 3-1/2" tall which is excellent considering I am only 5’-1/2" tall myself and my husband claims to be 5’ 8" tall, but is probably a little shorter.  All the family is short except my son who somehow ended up 5’-11" and my 2 nephews who are 5’ 11" and 6’ 4" tall.