you wait for your period to come on in may, if he doesnt then you take a pregnancy test.

actually, Plan b will mess with your cycle pretty badly. You may not have a period, have irregular periods/bleeding, or have very heavy periods for as many as 3 months (cycles).
Other than that, there is not much else you can expect.
If you'd like, you can take a test and get an accurate result 17-18 days after unprotected intercourse.

I took Plan B twice. Don't freak out when you don't get your period, mine was about 5-6 days late when I took it the first time. But, just so you don't worry, grab a pregnancy test. Most of them you can take 5 days before the start of your period