Can fexofenadine cause ringing in ears and how long for after you stop taking it?

Theoretically possible but unlikely. Occasionally it can be associated with cold symptoms and/or an ear infection causing ringing in the ears. If you otherwise are feeling well, then consider stopping the medication for about a week and seeing if your symptoms clear up by then. If yes, then it might be from fexofenadine, try a different antihistamine. If no resolution, get your ears checked.