
As with us ladies you count from the first day of one to the first day of the next one.
My girls have gone anything from 20 weeks when they first started their seasons, to the more usual 27 weeks, and as long as 32 weeks between seasons.
Breed often makes a difference. I understand that GSD can have quite short inter season intervals, as little as every four months. A friend had Welsh Springer Spaniles, and they cae in season on average every 9 or 10 months!