Syphilis itself typically causes symptoms--usually a sore on or near the genitals, anal area, or sometimes the mouth--3 weeks to 2 months after exposure. But the incubation period for other STDs is all over the map: as short as 1-3 days for gonorrhea or herpes, typically 7-10 days for chlamydia, and often several weeks for genital warts and certain other infections.
Also, most STDs most often cause no symptoms at all, or such mild ones they aren't noticed. So going (say) a few weeks after exposure without symptoms is no guarantee a persons isn't infected. If you had risky sex, especially with a partner you don't know well, go get checked. If you don't have a regular health care provider, call your local health department.
Good luck-- HHH, MD
Thanks for the info!
Regards,
Phil