Eighty-Sixed
Good question. I don't believe I've seen anyone ask about the longevity of a trespass warning before.
Of course, if you mean by 86ing, the casino informally telling or requesting the player to leave or is "welcome to play any other game but blackjack", there is no statute or time period, but failing to meet the casino's expectations could result in a formal warning.
Now, for the situation in which a person has been formallly read a State's Trespass Act or Trespass Warning, I checked Rose & Loeb's Blackjack and the Law. They say nothing about how long a formal trespass warning is effective.
Provided below is a link to Nevada's NRS207.200 which covers their statute about trespassing. It shows how a person may be told not to trespass - warned by the owner not to trespass or enter, signs, fences, hedges, etc.
Note that the statute does not provide a time limit. I draw the conclusion that if a person is warned by an agent of the owner, not to trespass, that the warning stays in effect as long as that owner has the property and can provide evidence of warning the person.