For some reason, this was cuff off my previous reply. Too long maybe!
No one knows exactly how long someone can have vaginal trichomoniasis with few or no symptoms. We do know, women can have this infection for at least three months without symptoms of any kind, and we know that it can be transmitted to a sex partner even when it is causing no symptoms.
As you mention, the diagnosis of trichomoniasis can raise questions for couples about their partner’s faithfulness. Although trichomonas is always transmitted by sexual contact, the fact that trichomonas can survive for several months, maybe even longer, means that an infection could have been acquired in a previous sexual relationship that ended several months earlier.
Also, men can have trichomonas but don't usually have symptoms. It's not clear how long men may have trichomonas, so making a diagnosis of trichomonas infection in men can sometimes be more difficult.
Hope this helps!