For how long do I have to live in Illinois before I can file for Divorce there?

Illinois does not let out of state residents file for divorce here. Illinios mus be your residence (you have to live here) at the time that you file for divorce. Also, at the time that your divorce is made final, you have to have maintained residency in Illinois for at least 90 days "next preceding the commencement of the action" or at the time that the court makes the finding of irreconcilable differences. In other words, we have a 90 day rule. As to what Illinois county you have to file in, that is called "venue" and is more about where you live than how long you have been there.