You wouldn't find a definitive expiry date on watercolour paper, the longevity of watercolour paper will depend on how it is stored and how the paper is made: Wood-based artist quality watercolour paper will have acidic components removed from the pulp to prevent it from going yellow or deteriorating. Many are also buffered with calcium carbonate to help maintain a neutral pH. Even so, 100% cotton watercolour paper is much more durable than wood-based paper and will remain in good condition for longer. With proper storage, watercolour paper will last for a very long time but it is very sensitive to environmental factors. Many watercolour papers are sized with gelatin, which can attract mould or insects if stored in a humid or hot environment. Humidity can also cause the size to deteriorate so care must be taken to ensure that the paper is stored in a cool, dry place.
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