If a duration is randomized, you oculd use cards instead of dice, so that
neither the players nor the GM will know the duration until it expires.

The number of cards you need is the product of the number of sides of the
dice; for example, if the duration is 2d6 then you will need 36 cards. One
card must be somehow indicated as the expiry card (e.g. the joker).

First, you must mix up the cards, without looking.

After each interval, draw the number of cards according to how many ways
there are to make that total by that many dice, e.g. there are six ways to
make seven by two six-sided dice, but only one way to make twelve, and you
cannot make one with two dice at all. Once you find the expiry card then
the duration has expired; then you do not need to draw any more cards.

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Don't laugh at the moon when it is day time in France.