Fexofenadine's half-life is 14.
4 hours, i.e., the amount of time for it to lose one-half of its activity. After 10 half-lives, it would be pretty much out of your system (only 1/1024 of its original activity). That equates to 140 hours or approximately 6 days. Allergists like to have patients off antihistamines like this for 1-2 weeks before doing any allergy testing.