When does borrowed car become stolen car?

I am a lawyer in CT and practice in this area of the law.  Embezzlement is the entrustment of property in another and the violation of that agreement with the intent to permenantly deprive the person of their property.  Larceny by trick is where a person induced another to give him their property and then never returns the property.  You situation can fall into either category, but most likely is an embezzelment.  The time period at which you may report the crime is the day after the person failed to return the car.  i suggest calling the person and if they fail to return your car in 24 hours, call the police.