Hi Robert,

Normal indoor conditions should be fine for storage. As long as they've been kept away from excessive heat, they should be good for around five years or even more. Attics and cars, for example, can get very hot during the summer and can dramatically shorten a ball's life. 

Cheers,

Mike

What about cold?  If I left a few dozen balls in my car during the extreme temps we've seen here in Boston this year, are they ruined?

Thanks! Can't wait for this snow to go away so I can use them.