How long can you go back to call a dead person "the late so and so?" JFK is sometimes called the late John F. Kennedy but I don't think Abrahm Lincoln is called the late Abraham Lincoln. Are there any rules there?
Garner in Modern American Usage (p499) has an excellent entry on this :
late. A. the late. This expression is elliptical for lately (i.e., recently) deceased. How long this can be used of a dead person depends on how recently that person died, but anything more than five years or so is going to strike most readers as odd (e.g., the late John F. Kennedy). Of course, there's no absolute statute of limitations; the question is whether a fair number of reasonable readers would know or need to be reminded that the person has died. But the expression offers more than just a reminder. It also offers a note of respect - and perhaps even a touch of sorrow. So in the fall of 1997 people said the late Princess Diana not because anyone needed to be reminded that she had died in August of that year - everyone knew it - but because people mourned her death. By the same token, a widowed spouse might continue to use my late husband or my late wife.
Edit: The above answer has now been added to the original (identical) question at: How long can you say "the late so and so"? .