The Copyright Act in the US sets the statute of limitations at 3 years and there are no State copyright laws so that is the only deadline that is in effect in the US. . The safest starting date to use is the date on the first Getty letter you receive. Three years from that date is the latest possible deadline by which they would have to file a lawsuit. Also, when they claim that they are seeking damages because your image was up for the past 6 years, they can only obtain damages for three years prior to the date of the letter up to the point you removed the image.