Pregnancy test - if I have sex and conception happens, when after the sex act will the test show positive results? (days, weeks?)

2 weeks. If you have sex near the time of ovulation and you get pregnant your pregnancy test should be positive about 2 weeks later.
Pregnancy tests. Most pregnancy test are very reliable at the time of a missed menses. If your period is late, a pregnancy test should reflect whether or not you are pregnant. Prior to that time, they are not really reflective.