they don't expire as far as I know....been collecting them for a yr. every now and then our local school has a contest w/them so I share w/the grandchildren. so I guess they arre oooook. how do you find out? they are not dated. these are off goods, not coupons
They should have a date on the. Look at the image you posted from the cereal box. At the bottom of the label it says "expires 6/1/17". that's when it expires and is no longer valid. Generally they are good for a year or so after the date you purchased the item from.
Plus, the schools won't tell you if you sent in expired labels. They just toss out the bad and keep the good :)
Also, the 'bonus' box tops (the big ones worth 3 or more) have expiration dates too. Again, towards the bottom, and in smaller print.