I overdosed on entex yesterday; how long does it stay in my system?

Depending on amount: I certainly hope you just took a bit more than what is recommended and not intentional od. Anyhow, entex has 2 meds in it and phenylnephrine has half-life of about 3 hours (time it takes for 1/2 to disappear from your body). Usually there is much left after 5-6 half-lives (15-18hours). If you have bad symptoms, please consult doc. Becareful using any med; some can have devastating consequences. Good luck.