Sometimes shorter and sometimes longer. Usually every month. When a girl first gets her period it may not come every month. It may come irregularly for the first year or two and then start to come more regularly. The average cylcle is 28 days but some girls get their period every 35 days or every 21 days and this may be normal for them.
Some girls even get their periods at different times each month.
Each of these cycles may be normal for that girl. A. Ovulation usually occurs 14 days before you get your period. In a girl who gets her period every 35 days you can predict that she is ovulating on day 21. In someone who is irregular and has a period 28 days one month and 21 days the next and 35 days the next, it is hard to tell when she is ovulating. If you have irregular periods you may not be certain of when you are ovulating.