Well from what I know, and I researched it extensively after someone I know was in a fatal car crash (they smoked the night before) I found that when any substance is in the blood stream then you are under the influence of that substance, as soon as it leaves the bloodstream then you are no longer. After leaving the bloodstream thc becomes stored in your fatty tissues and is eliminated slowly through urine.

The person I know that this happened to was in a fatal car accident at around 7AM and she had smoked around midnight the night before. Anytime there is a fatality involved they automatically take blood and test it for drugs and alcohol, so she was really freaked out about it. The test came back negative thank god.