What should I do if I accidentally take too much allegra (fexofenadine)?

Call poison control. It depends on how much you take, and what other medical conditions you have. When in doubt call poison control, contact your doctor, or go to the emergency room.
Stop taking it. The half life of Allegra (fexofenadine) is 14.5 hours in normal individuals. It is metabolized mainly through the liver and GI tract so if you have liver disease it may be metabolized slower. This means that in a normal individual about half of the medication is left after 14.5 hours.