The average time between heat cycles in the dog is 7 months though this can vary widely. Though dogs are considered not to have seasonal estrus (heat depending upon time of year) the highest estrus activity in dogs occurs late winter and spring.
My female had a heat 3 times starting at 4 months old, however my vet informed me this is very rare. Most times 2 times a year is the norm, 8 and 4 month cycles or 7 and 5 month cycles. I think the myth they go every 6 months like clock work is just that a myth.