Periods typically occur about every 28 days on average, but a normal menstrual cycle can range anywhere from 21 to 35 days. The menstrual cycle is counted from the first day of one period to the first day of the next period. Periods generally last between 2 to 7 days, with most lasting about 5 days. These variations are normal and can differ from person to person. The cycle length and period duration may change slightly due to age, health, and other factors like pregnancy or hormonal changes. It's also common for cycles to be irregular when periods first start or as menopause approaches. In summary:

• Average cycle length: ~28 days (range 21-35 days)
• Typical period duration: 2 to 7 days (usually around 5 days)
• Cycle is from first day of period to the first day of the next period

This pattern reflects a typical menstrual cycle and can vary individually.