I know they can start as early as 6 months but I'm not sure the longest they can go. they usually have 2 heats a year. There is supposed to be swelling and red discharge in the 1st stage of heat. that said I'm not sure how much swelling, and some dogs are supposed to keep themselves really clean. I'm not sure how noticeable it is.
Highdee If you're considering breeding be sure you read up on it. I thought it would be neat when I first started looking for a yorkie mostly because I love the idea of having puppies around, but changed my mine when I read about the risks and what all went into it. I decided I just wasn't up for that... I know a lot more goes into it than I would have though.
I'm not a breeder but from what I remember you aren't supposed to breed the 1st heat, and from what I've read on spaying spaying before the 2nd heat greatly reduces the chances of certain types of cancer in dogs. There is a lot of great information here
