
Being female, and this is how it is done in people, you count from the first day to the first day!
As has been said they vary. Anything over 4 months is considered normal! Any less and there is an imbalance of some sort with possibly cystic ovaries or non ovulation.
It is my understanding that GSDs often have short inter season durations. some coming in season as often as 4 monthly. In Welsh Springers it is often about 10 months.