A normal menstrual cycle is the gap between 2 day one of periods is 21 to 37 day Bleeding including spotting during periods should not exceed 7 days Beyond this it's an irregular cycle

You should first start getting regular periods. So many girls at the age of 19 years may not have established regular periods. Once you get regular periods ( at least 6 months of regularity as far as amount of blood loss, duration of flow and interval between two periods) , then any deviation from anyone of these factors will be considered as irregular periods.

When it doesn’t follow a regular pattern