I was diagnosed with strep throat and just started on penicillin today. How long does strep usually last when it's being treated?

10 days. The sore throat usually gets better in a few days but the strep is not eradicated until after 10 days of penicillin treatment. The pharyngitis usually resolves within 7 days of onset even without treatment, but unfortunately there is then the risk of glomerulonephritis and rheumatic fever so be sure to complete your full 10 days of treatment even if you feel cured after a few days.