Your menstrual cycle is counted from the first day of your period up to the first day of your next period. The typical menstrual cycle is 28 days long, but each woman is different. According to the Office on Women’s Health, a normal menstrual cycle ranges from 24 to 38 days.

Cycle variation is an indicator of cycle regularity. Within the same year, the length between the longest cycle and the shortest cycle can vary up to 9 days and still be considered within normal range. Factors like stress, diet, exercise, or hormonal imbalances, can sometimes affect cycle length and cause irregularities. If your period change is sudden, don't wait longer than 3 months to talk to your doctor.

Reference: 

1. Office on Women’s Health, U.S. Department of Health and Human Services. (2021). Menstrual cycle. Available at: https://www.womenshealth.gov/menstrual-cycle/your-menstrual-cycle (Accessed: 06 June 2022).

2. Munro, Malcolm & Critchley, Hilary & Fraser, Ian. (2018). The two FIGO systems for normal and abnormal uterine bleeding symptoms and classification of causes of abnormal uterine bleeding in the reproductive years: 2018 revisions. International Journal of Gynecology & Obstetrics. 143. 10.1002/ijgo.12666.