Cycle length is measured from day 1 of bleeding to the day before your next period starts, not from the end of one period to the next. Anything in the roughly 21-to-35-day range is generally considered regular for adults, and slightly longer cycles are common in teenagers and in the years approaching menopause.
It is normal for cycle length to vary by a few days from month to month. What matters more than a single perfect number is your own usual pattern. Tracking a few cycles with an app or calendar gives you a baseline so you can notice meaningful change.
See a doctor if your cycles are consistently shorter than 21 days or longer than 35 to 38 days, if they suddenly become very irregular, if you skip several periods (and pregnancy is ruled out), or if bleeding lasts longer than about 8 days. These patterns can have many causes, from thyroid issues to polycystic ovary syndrome, and are worth checking.