Can someone tell me how long will strep throat last if left untreated?

2-5 days. Strep throat infections are self limited usually lasting no longer than 5 days. Rarely, the strep can go to another part of the body setting up a 2nd site of infection.The danger with strep is in the sequelae or aftermath of the infection, that in susceptible individuals can lead to rheumatic heart, kidney or brain disease. It is for these reasons that strep infections should be treated.
Problems untreated. Strep throat is a bacterial infection. It can easily be diagnosed with a rapid test and treated with antibiotics. Untreated, the symptoms can last 4-7 days. The problem with not treating strep is that significant complications can arise, such as rheumatic heart disease, kidney disease, and neurological conditions referred to as pandas. Please seek medical care if you think you have strep throat.