The menstrual cycle makes up most of the physical health of a woman. The cycle is influenced by many factors, and often times a woman can experience changes in their cycles.
A normal menstrual cycle is said to be maintained, if the time gap between two periods is roughly 21 days to one month!
Stress, anxiety, eating habits, medications, physical activities all effect the cycle on a huge basis.
The best way to track a normal menstrual cycle, is to note the gap in between 3 to 4 consecutive periods.
The gaps should not be more than a month, and you should be tracking it each month! Like say, if your period takes place on 1st or 2nd date of a month, then it should again take place somewhere in the first 5 to 10 days of the next month. A one week extra gap, is normal.
But if you notice severe changes in your cycle, like one month the period starts on the 5th, the 2nd month it starts on 15th, the next month it is skipped and it takes place on the 4th month again, then you should consult a gynecologist.