Is she pregnant? & how long delayed before we can consider that she's pregnant?

had unsafe s*x with my girlfriend on march 23.. and she told me today that her mens were already delayed for 3days as of now..

we're both nervous as hell..
so here's my question..

how LONG/MANY days she should be delayed before we should be worried & before we CAN CONSIDER that she's pregnant?

im 17teen & she's 18 :<

Enough time has passed to take the test. You can get a pregnancy test kit and answer the question now. They cost 5 to 15 USD and they're available at a pharmacy.

Pregnancy info:
http://www.plannedparenthood.org/info-for-teens/pregnancy/am-pregnant-33831.asp