Very little. Neodymium magnets are thestrongest and most permanent magnets known to man. 

If they are not overheated or physically damaged, neodymium magnets will lose less than 1% of their strength over 10 years - not enough for you to notice unless you have very sensitive measuring equipment. 

They won't even lose their strength if they are held in repelling or attracting positions with other magnets over long periods of time.