Do you mean how long after a missed period will a pregnancy test accurately come up with either a positive or negative result?
It depends on how "pregnant" the woman is (hormone levels). I once had a positive result only 8 days after ovulation (yes....we were tryin' REAAAAL HARD).
A false NEGATIVE is likely.... a false POSITIVE is highly UNLIKELY.
Sorry to hear about your trouble. If it is a good thing, then .....GOOD LUCK!