About 2-5 weeks. Treponemal antibody tests, like fta-abs and mha-tp usually become positive two to five weeks after the initial infection. These tests stay positive, even after therapy. The vdrl and RPR probably become positive before then. The fastest test would likely be the darkfield exam, but that needs a sample from the chancre, or fist lesion, and needs to be done 10 minutes after getting the sample.