When you refer to the deceased, you say "the late so and so." How long can you say that? Is JFK referred to as the late John F. Kennedy? How about Abraham Lincoln?
There is no set limit to how long one can refer to a deceased person as late.
The consensus of opinion seems to hover around 15–30 years. However, if the person is unknown to the audience, one can use it for much longer (my late husband can refer to someone who died 50 years ago).
The late William Safire (December 17, 1929 – September 27, 2009), American author, columnist, journalist, presidential speechwriter, and all-around expert on speech, suggested 15 years.
Edited to add references and limerick.
There was a young man in a car,
Who said to his father, "Papa,
If you drive at this rate
We will surely be late,
Drive faster!" He did, and they are.
From: Harper Dictionary of Contemporary Usage, Second Edition (1985) William and Mary Morris: There is no precise time element involved in determining how long a person must be dead before he is no longer considered late. As a general rule, late is used in reference to persons whose death has occurred within the twenty or thirty years just past. On the other hand, it is proper for an elderly person to refer to a contemporary who has preceded him in death as "the late."
Webster's Dictionary of English Usage (1989): ...Here are a few opinions: ...the statute of limitations might run for half a century" --Bernstein 1971 "As a general rule, late is used in reference to persons whose death has occurred within the twenty or thirty years just past" --Harper 1975 "... 'the late' is used for about ten to fifteen years after death" --Safire 1984
There is no limit, it's just a matter of style. It kind of seems superfluous at some point, especially if you're speaking of somebody everybody you're speaking to knows is dead and has long stopped grieving.
EDIT: This is a point of style, not correctness. The one time I might use "the late" when referring to a long dead person is when I'm specifically trying to emphasize the tragedy of the death. For instance, if I were speaking to The United States Secret Service about the folly of convertible tops in cars used for Presidential motorcades, I might refer to JFK as "the late".
The Oxford English Dictionary’s definition 5a of late ‘Of a person: That was alive not long ago, but is not now; recently deceased.’ Just what period of time ‘long ago’ and ‘recently’ refer to is a matter of judgement.
In the cases of Lincoln and Kennedy, it’s not necessary to use it, because everyone knows they’re dead.
The following entry by Garner in Modern American Usage (p499) addresses several of the points made by previous answerers and commenters on this question:
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.