How often do dogs have their periods?

well i had abitch labrador she came into season (had periods) twice a year. and poor dog used to get bad pmt she would have a fit just before one the vet said just bad pmt bless. the 1st season was about 6 months after she was born then every 6mths my dog had them in may and November they were regular i had to watch her about 4-6 weeks prior as she would run off and leave me stranded on my own . i suppose her hormones told her she neede a male dog bless but i allways knew the right time to keep her on lead any way i do hope this answers your question, how bigs your dog?? its agood idea not to let her on furniture be4 this as it can be quite messy again depending on what type of dog u have and they are allways licking down there good luck