[~*Nessy*~]

Can you change the number of days your cycle lasts on FF?

it will determine the cycle length based on when your AF starts and when your next cycle begins

[MamaAnna]

The first day of full menstrual flow even if only light will start a new cycle. If you change that date it will adjust the cycle days. Keep in mind that you do not want to alter that date if you did indeed have full flow.

[~*Nessy*~]

For some reason I thought that I set the cycle length.

[MamaAnna]

No, FF keeps track of it which will help it make predictions. By collecting the information itself it will better be able to tell you what days you are normally fertile, how long your longest, shortest and average cycles are, etc. You could get by doing nothing more than entering your temp each day and FF would do the rest.