If I was fired from my job and accused of stealing over 3 months ago but have not yet been charged, how long do they have to charge me?

You may want to speak with a criminal defense attorney in your local area, but typically there is no standard time limit for a prosecutor to  charge a person if they believe probable cause exists to show a crime has been committed. With shoplifting, it is not unusual for a citation to take some time to issue. You can always contact the court where you worked to see if a warrant has been issued against you.