If I treat myself for a yeast infection but have an STD how long will it take symptoms to reappear?

Few days. better to have it checked out for std tests so you would not miss and not have short or long term suffering.
STD. If you actually have an STD, the treatment for a yeast infection shouldn't make the symptoms go away. STDs, however, are not always associated with symptoms, so you should get tested regularly regardless.