the average is 28 days, but every body is different. the problem comes in when you have a drastic change in the amount of days, then there could be issues
All women are different and can have different cycles. Some women have a period every 28 days, some every 30 days or so and some are irregular either all the time or most of the time.
Are you having problems with yours? If so, you may want to see your Gynecologist.