As with humans you count from the fiorst day of one cycle to the first of the next. Wish it was six months as I have had my youngest come in after 16, 20 and 18 weeks. :( Hoping she doesn't come in until end of September as she came in in the middle of April.
Bitches are all different, but count from the first day of season. I had one bitch that was 6 months to the day, her daughter who went 10months to the day, and another daughter who went anything from 39 weeks to 19 weeks!!! Hated trying to organise the mating of that one! ;)