Female dogs stay in heat, technically known as "estrus," for five to 13 days. The length varies between different breeds and individual dogs. Male dogs are attracted to female dogs for approximately 18 days.
Female dogs usually go into heat twice a year, with the first cycle occurring on average between 6 and 12 months of age. It can occur earlier in small breeds or later in large breeds. Signs a dog is in heat include increased urination, a swollen vulva and a yellowish vaginal discharge. About.com recommends spaying female dogs before their first heat unless the dog is intended for breeding.