How long do one shot of blood thinner stay in your system ?

Shot?: I assume you mean one of the injectable heparin compounds such as enoxaparin? The clinical effect could last anywhere from 12 to 24 hours depending upon dose.

I assume: you mean enoxaparin? It has a half life of 4.5 hours. In order to completely eliminate a drug, 5 half lives have to pass. This means 5x4.5, or 22.5 hours. Almost a day. Hope this helps!