Longer, actually.
You just bombarded your body with a very high dose of hormones. Your body has to cycle through those hormones and work them out of your system.
Here are the side effects as listed on the Plan B site. As you can see, it can even affect your next period. For me, I skipped a period completely and had a very heavy for the next month.
Side effects:
When taken as directed, Plan B One-Step® is safe and effective, and the side effects are generally mild. Some women may experience side effects similar to those of many birth control pills, including changes in their period, a heavier or lighter next period, or a period that is early or late. If your period is more than a week late, you may be pregnant. If you have severe abdominal pain, you may have an ectopic pregnancy, and should get immediate medical attention.
When taking this product you may have:
Nausea
Lower abdominal pain
Fatigue
Headache
Dizziness
Breast tenderness