Strep Throat - What Happens. Symptoms of strep throat usually begin within 2 to 5 days after you come in contact with someone who has a strep infection. Strep throat usually goes away in 3 to 7 days with or without antibiotic treatment. In contrast, if allergies or irritants are the cause of your sore throat, it will usually last longer unless the cause is eliminated. If strep throat isn't treated with antibiotics, you will continue to be contagious for 2 to 3 weeks even if your symptoms go away. You are much less contagious within 24 hours after you start antibiotics and are less likely to develop complications of the strep infection.