I think he means, "If I accidently injected into an artery, how long would it take to know?"
You would know right away, especially after you pulled out the 2 inch needle needed to hit an artery; most arteries are pretty deep. Now a vein, that is more likely, and most likely you would end up coughing quite a bit shortly after doing it - as well as it would most likely gush blood when you removed the needle.