The average duration which a dogs season lasts is three weeks (21 days). However, this season can be short (two weeks) or long (four weeks) depending on the breed of your dog. Some of the signs which may help in establishing whether your dog is on heat are: swelling of the vulva, bleeding from the genital area, increased urination, and 'change in personality'.
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Depending on your dog's breed, age and size, her season can last for anything between 2 to 21 days, although it lasts for 9 days in most dogs. She will be in a stage called pro-oestrus before she comes in season, which will last for a maximum of 11 days. Your dog's behaviour will change during this period, so you need to be patient with her.
The heat period in a dog varies from 8 to 30 days with an average of about 20 days. Most bitches will allow breeding around 10 to 16 days after start of the cycle. You should consider your dog to be 'in season' for 21 days, that is, 7 days coming into heat, 7 days in heat, 7 days going out.