How many weeks does it take to know that your pregnant?

This will generally depend on the test that is used to detect a pregnancy.

Blood tests are more accurate in that they can detect a pregnancy earlier. As soon as two weeks after a missed period, blood tests can detect levels of Hcg (the hormone pregnant females produce).

Urine pregnancy tests generally do not detect a pregnancy before the 6th week because sometimes it takes that long for Hcg levels to rise to the point where a urine test will be sensitive enough to detect it.