How long do you have to run from the police before they cant charge you?

You can be arrested and charged for any attempt to elude or escape, regardless of the length or brevity.

Additional: (in the US) every state has some kind of Criminal Statute of Limitations, past which certain enumerated crimes cannot be prosecuted - and not ALL criminal offenses are exempted. They vary widely from state-to-state and you would have to check your own state's SOL's for more specific information.