There are some pregnancy tests you can buy that allow you to test around 3 days prior to your period, & claim they can give you an accurate result.
For most pregnancy tests, you need to wait until you've missed the first day of your period.
Because of the amount of Plan B hormone in your body, you're not going to know when your period would normally have come. Therefore I'd suggest you're not going to get a reliable pregnancy test result until you've waited at least 3 or 4 weeks after the date you had sex, as a rough guide.
You really need to sort out a better method than Plan B for your birth control. It's not good for your body. You don't know what problems it may cause you in the long term. It just leaves you with weeks of stress about whether or not it worked, & weeks or months of either no period, irregular periods, or heavy bleeding.
I've been there, & it's just not worth the stress of worrying if you're pregnant. That stress can even cause your periods to be irregular.