Menstrual Period, Late or Missed. A menstrual cycle is the time from the day a menstrual period starts to the time the next period starts. The average menstrual cycle is 28 days long. However, a normal cycle can be shorter or longer than this. It may be anywhere from 21 to 35 days long. Most periods last 3 to 5 days, but anywhere from 2 to 7 days is normal. Menstrual cycles may happen around the same date every month or they may be irregular. A menstrual period is considered late if it hasn’t started 5 or more days after the day you expected it to start.