How long can you say "the late so and so"?

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

You can say it for as long as you like, they're dead forever. The late X is a euphemism for the dead X.

People stop when the fact that someone is dead is general knowledge, but they don't have to stop.

It's useful if there is a living famous person with the same name as a dead famous person.

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".