4yr (2yr to menses) A girl's puberty lasts about 4 years overall, but the range can be from 1.5 to 8 years' time. Her periods start about 2 years after puberty begins. The later her puberty starts, the shorter the time between the beginning of puberty and the start of her periods. So, a girl who starts puberty at 9yr might have her period 2yr+9m later, but a girl who starts puberty at 13yr may have a period 6m later.
Five years. Generally puberty lasts about five years. The very first signs such as underarm hair or body odor start to develop in elementary school or junior high. Then the growth spurt starts. By the time a girl gets her period she is closer to then end of puberty but may continue to grow for a year more.