Some dogs are like clockwork, every six months to the day. Other dogs only come in once a year. Some will miss a cycle and then go back to every six months... You will just need to figure out your dog. First heat will usually come in between six and 12 months, but I have had one that wasn't until after that and then waited a year before the next and then went to six months like normal. Go figure! I try to write down on each dog's folder when I first notice blood dripping so I have a record. I have had an accidental breeding before. It was something that I was planning on doing anyway, but I was going to wait another cycle before trying it.