Peterx60,
Depends.... 30 years is the norm, but there are some documents that have 50, 75 or 100 year classification upon them.
Some documents will be 'weeded' prior to release too. Not sure how FoI might affect this.
And that all assumes that it was written down in the first place! I'd guess, for accounting purposes that the loss of a Firestreak from the inventory would have to be recorded, but of course, if recorded as an emergency....