6-12 months Three weeks or around 21 days.
The average heat cycle for a dog is approximately 3 weeks and since this an an average, some heats are shorter (as little as 7-10 days), others are longer (4 weeks or more). Average times between heat periods is seven months but some dogs can cycle as early as every 4 months, some once a year. As the female starts to go out of heat or enter diestrus, she will be less willing to breed. Again this stage can last 4-14 days but averages approximately 7 days. The next cycle usually begins about 7 months from the start of the last heat cycle, not the end of that cycle but again this varies from dog to dog. A female dog will generally stay in heat for about three weeks or around 21 days. However, this heat cycle will vary from one dog to the next. It depends on the breed, the age and the dog herself. A female dog will go into heat twice a year.  Dogs are usually in heat for 21 days. Female dogs have a cyclic sexuality that is independent of season unlike most animals. As for the male dogs, they do not have any cycles and are always ready for intimacy like most males of any species including human beings. This stage lasts for ten days and once it is over the dog will remain out of heat for the next 6-8 months. Further reading: http://ianswerit.org/2013/11/17/long-dog-stay-heat/. Timing. Your dog can come into her first heat any time between 6 and 24 months. According to Webvet.com, small dogs come into heat sooner than larger breeds. Typically she will have two estrus cycles, or heats, a year, and each one lasts for between 18 to 24 days. She is likely to show signs that her heat is about to start by changes in her behavior. While for the large breeds, it can start when the dog is about 2 years old. Female dogs normally have only 2 reproductive cycles in a year. During the cycle, they will experience bloody vaginal discharges that last for 4-9 days. After this duration, the female dog will begin to stand for breeding and accept the male.  Expert: Gina replied 8 years ago. A dogs heat cycle will last for a full 3 weeks..the first week her vulva will swell and she will have a red blood discharge..the second week the red discharge will turn a straw color and the third, the swelling will start to go down and the discharge will start to run clear.  1 On average, dogs have two cycles a year. 2  The estrus cycle lasts on average 12-21 days, but maybe be as short as a few days to four weeks. 3  The estrus period length varies widely between breeds and individual dogs. 4  The length of a cycle varies widely, even for dogs of the same breed. 1 The estrus cycle lasts on average 12-21 days, but maybe be as short as a few days to four weeks. 2  The estrus period length varies widely between breeds and individual dogs. 3  The length of a cycle varies widely, even for dogs of the same breed. 4  If in doubt, assume the longer end of the range for the cycle length. Hi Winnie, Female Dogs go into heat at any time between 6-12 months usually. The average heat cycle for a dog is approximately 3 weeks but for some dogs it is longer and for others it is shorter. Some as short as 10 days and some as long as 4 weeks or more. 
http://www.labradornet.com/heatcycle.html http://www.pethealthandcare.com/askquestion/3119/how-long-does-a-female-dog-stay-in-heat.html http://dogsonlinehelp.com/how-long-does-a-dog-stay-in-heat/ http://pets.thenest.com/long-bleeding-phase-last-dogs-heat-4132.html http://www.labrador-retriever-guide.com/dogsinheat.html http://www.justanswer.com/pet-dog/0kg80-long-does-young-female-lab-stay-in-heat.html http://vetmedicine.about.com/od/pregnancybirthindogs/f/k9_estrus.htm http://www.justanswer.com/pet-dog/0o2f2-long-does-black-lab-s-bleeding-last-when-heat.html
how long does a female lab dog stay in heat