Well, some people have said that their thrush showed up as early as 5 days of coming into contact w/ someone who they believe already had thrush, or a yeast infection ~ really, it can depend on each person's oral hygiene habits, or lack thereof; and possibly daily diet... Basically, the cleaner you keep your mouth, the less likely you are to have bacteria grow (mouth rinse, floss, & brush at least 2x daily) ...
Regarding your BF and the non-prescribed med.