Usually the first "heat" or "season" last about 14 days. There is a period of fertility during the second week, this is when there is almost no bloody discharge, and the female is most potent. But keep track of when she does start then count six months, and this will tell you when she should start again. Most dogs usually have only two "heats" per year.
As for spaying, it is recommended to have her done BEFORE her first heat. At six months. The common reason is due to increasing populations of unwanted puppies, but another good reason is breast, uteran and ovarian cancer in dogs. Some suggest allowing the female to go through her first heat, but I disagree. If you are planning on spaying her, do it before. She will be much better off for it, and so will you.