All dogs have a cycle of approximately 3 weeks. They are not ready to mate until about 4-7 days right in the middle of the 3 week cycle. So that's the middle week. When she shows signs of her vulva reddening and swelling that is when she is most ready.
That's true -she will definitely show the male if she is not ready. She will snap at him if he tries anything. If she does that, don't scold her. Doing that is natural, won't do him any harm. When she snaps at him, he will usually back off and wait.
Dogs will mate towards the end of her cycle. I know larger dogs have a cycle of about 3 wks. I'm not sure with smaller dogs. If she isn't ready to mate she will let the male know.