Very little. Neodymium magnets are classified as the strongest and a lot permanent magnets known to man. If they are not overheated or physically damaged, neodymium magnets will mislay a lot less than 1% of the strength over 10 years - too little that you notice if you have very sensitive measuring equipment. They don't even lose their strength when kept in repelling or attracting positions with other magnets over long periods of time.

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