Steaming can not be relied upon as a method of birth control. Many people steam after ovulation, not yet knowing they conceived, and do not experience pregnancy loss.
If one were to attempt the use of vaginal steaming to prevent potential conception after unprotected sex, they would need to steam as soon as possible after insemination for three times a day with cleansing herbs for at least three days in a row for 45 minutes each steam session.
Please note, however, that steaming with this frequency may cause side effects and will very likely cause unexpected period changes for most people. It is best done under the guidance of a practitioner trained in this area.
Also keep in mind that there is no guarantee that this steam plan will prevent pregnancy or initiate a period.