When should I take a pregnancy test for an accurate result?

Last month I took plan b because the condom broke. I got my period on time. This is my second cycle since taking plan b. I usually use condoms or the FAM method. This time I used FAM & had unprotected sex on day 20 of my 25-30 day cycle. Today is the 30th day of my cycle & I am freaking out because my period is no where in sight. I took a pregnancy test yesterday but it was negative. When should I take a test & be able to trust the results?