How long does it take for her to know if she's pregnant?

She has to miss a period.

Like Nauticalstar420 said, in order for pregnancy to be suspected, a missed period is the first substantial clue. She could have had symptoms before, but can't be taken as a sure sign, as symptoms for pregnancy are the same symptoms for an upcoming period, being sick, and many other things. Even after a missed period, it isn't always known for sure as some women don't have enough hcg (pregnancy hormone) built up in their systems to register on a hpt(home pregnancy test) immediately after a missed period, and they have to wait a week, or two, or more before knowing for sure. The earliest you can get a pregnancy result on a hpt is 5 days before the missed period. (again this can be wrong because of the need to build up enough hcg, so not all women get an accurate result with these)

The above is all true..,. but to explain it so that you could understand better (seeing as you are male and don't have aperiod, and therefore don't know the time frame involved as well as women do)...

A cycle is typically 28 days long the first 5-7 days are when the woman menstrates. Day 1 of the cycle is the first day of a woman's period. So by days 12-14, typically a woman is ovulating and Most fertile. If, (and I say if because you can get pregnant anytime during your cycle, even during menstration) an egg is fertilized during this time it will take as long as a week to 2 weeks for the egg to implant and start producing Hcg.(typically by the time her next period is due). But the levels have to be high enough to trigger a test for it to show positive. Fertilization and implantation can occur at the very beginning or very end of the cycle, and therefore can change everything. The woman could get a positive test before her missed period, or very long after, depending on the hormones and time of conception. But regardless at this stage, until an ultrasound proves different, When determining the age of the fetus you go by the date of the LMP (Last Menstrual Period) and therefore lets say, if your partners last period was June 1st, then the age of the fetus would be approximately 3 weeks tomorrow, even though fertilization would not have likely occurred on June 1. It is just the way the medical field determines the age of a fetus, since the window for conception is so large... Does that make sense?