to be exact, I meant to say that a 6 week test for HIV is fairly reliable, 3 months for herpes etc.
It varies quite a lot depending on the disease in question and who gives the opinion.
I would say that after 2.5 weeks chlamydia and gonorrhea are the ones that can be seen with a test, and most likely they would have caused some symptoms as well. In addition to this, also herpes would most likely caused some symptoms within 16 days after possible exposure.
Where I live (somewhere far) they recommend doing hiv, syphilis, hepatitis and herpes blood tests 3 months after possible exposure, however, dr. H has mentioned that in his opinion 6 weeks is pretty reliable already, and naturally if you are really concerned, it does no harm to make a test earlier, but do another after 3 months.
Check http://www.cdc.gov/std/ for further information.