15-18 months here in the North for the first heat. I also find daylight affects heat cycles. When I have a girl due to come in during the winter, I keep the fluorescent light in my mudroom on until 10 pm. When I did not do this - heat cycle would be as late as 9 months after previous cycle, but when light is left on they cycle every 7 months.
(I also find keeping light on keeps pigment of nose blacker mid winter.)
I think it depends on lines... my oldest came into season for the first time around 8.5 months of age. I have heard that her daughters have come in for the first time as early as 6.5 months. My yellow girl came in for the first time at 15 months of age, her dam came in at 14 months for the first time. I have a female puppy that is young so it will be interesting when she comes in for the first time :)