After regularly testing negative for syphilis every three months, I began having symptoms a month after a negative result (though I didn't recognise them as such) and by 6 weeks serious complications were already developing. My doctor had me test again and we got a positive result. In that total 8 weeks things moved very rapidly. in addition to the common rash, I developed ocular involvement (bulged optic nerve) pericarditis, a flare in my arteritis and colitis, nodes in my lungs, an ulcer in my rectum, and atrial fibrillation. Due to the ocular involvement, I went straight into the hospital for a round-the-clock penicillin drip followed by a home infusion, totalling 2 weeks of 24-hour penicillin.
Had we caught it when the symptoms first appeared, I might have gotten by with the simple shot in the butt. In my case, I rapidly had an inflammatory cascade that did a lot of damage in a very short time.