A severe sore throat can last anywhere from a few days to several weeks, depending on the underlying cause. Allergies are one of the most common causes of a sore throat and can be accompanied by other symptoms such as sneezing, itchy eyes, coughing, runny nose and fatigue. If your sore throat is due to allergies, it should begin to improve within a few days after starting an antihistamine or decongestant medication. However, if your symptoms persist for more than two weeks or worsen over time then you should seek medical attention as this could indicate another condition such as strep throat or tonsillitis.