Tetracycline & Birth Control

I had a flareup of rosacea last week and decided to start taking my tetracycline. I took one pill Tuesday morning. About an hour later, I remembered that it decreases the effectiveness of the birth control pill (I'm taking Enpresse). I did not take anymore pills after that as I was heading off on vacation. How long does it take for tetracycline to get through your system before it's safe to assume it will not affect the birth control pill? I'm a bit nervous that I may be pregnant.