Counting the days until your subsequent time...is it 21 or 28 days?

Do you count 28 days from the day of your later period, or 21 days. Or possibly its count 21 days from the first day of your second period. Or possibly its….oh, please help! Which is it? 21.
from the ruin of your period.
28 is including your extent
HOWEVER
this is just the average for adjectives women. I track my period online using www.mymonthlycycles.com so I know my cyle length, when I'm fertile, and when I am going to draw from my period.
So for me, my average cycle is 25 days. Its adjectives very personal. But 28 is solely an average.
28 days.