That depends on the length of your cycle. The average is 28 days although lots of women have shorter or longer cycles then that. If your average cycle is 28 days, and you started your last period the 15th of September, your next period would have been due to start the 13th of October, making you 12 days late. So in order to properly answer that question, we would need to know the length of your cycle and the first day your last period started.