Usually none. The majority of dogs do not have silent heats and do show discharge throughout their heat, approximately 21 days. Fertile time period varies by individual dog, but most are fertile anywhere from day 13-18. Usually they are not fertile anymore after this time, and once estrous is over, they cannot get pregnant.
It is not always best to just go by discharge to determine when a dog is and isn't fertile. Best bet is o have progesterone testing done for this.
Hope this helps,