I'm sure that there'll be a lot of answers for this but don't use marbles as they do nothing and may contaminate the whisky. I don't know where you heard that it goes bad as that's a myth, what happens is that there is a slight lessening of the flavour intensity, always assuming that you seal the bottle after you've taken a drink. Normally a bottle will last up to a year without a noticeable drop in flavour and if it's kept out of sunlight and away from heat could last much longer.
Just forget all the myths and enjoy it
Slainthe
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