Cats can have their first heat cycle between 4-6 months of age and will go in to heat approximately 3 times a year. Each litter typically has between 4 and 6 kittens. Dogs go into heat at about 4-6 months of age and typically have two heat cycles per year. Each litter has, on average, between 4 and 10 puppies. A dog’s heat cycle lasts a total of 30 days. The vagina will swell during the first 10 days. There will be bloody discharge during the next 10 days.