Who long does an sti starts to show?

That depends: The incubation period is different for each sexually transmitted disease.

Depends: Some like the cancer associated HPV may never show symptoms until cancer shows up (rare) decades later.Some people are not symptomatic with gonorrhea or chlamydia while they pass it to others.The women can become sterile before symptoms appear.If you are sexually active consider periodic STD screening as a necessary part of the process.