When something in a role playing game has a randomized duration, you may
wish to avoid knowing the duration until it expires. My suggestion is to
use cards instead of rolling dice. For example, if the duration of
something is "1d6+2 minutes", then take six cards, one of which is a joker
(or ace of spades if you do not have a joker); after three minutes, draw
one card, and draw another card after each minute (of time in the story),
and the spell or other effect expires once you find the joker.

One way of altering the probability distribution is to vary the interval
at which cards are drawn.

There are other ways, too. For example, take two cards of each rank ace
to king, and draw one card each turn, until you have drawn two cards of
the same rank (not necessarily consecutively).

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

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