Any time. Implantation occurs about 7 to 10 days after ovulation and a urine pregnancy test can be positive shortly after implantation. It is unlikely that you could know the precise time of implantation - so a good reference is to perform your pregnancy test at about the time your period should be due. If you are pregnant, the test will almost always be positive by then.
Depends. If you mean after conception, if is after you miss your first period. Some test detect hormone level rise within the first 7 days.