I don't think they expire, but if you have trouble, seriously, just ask, I've got way too many of them.

And I agree with you 100%, this feature would be ok for others who want to use it (though I would never use it in a million years) if it did not apply to other people's locked entries. Total invasion of privacy. I trust my flist not to use it on purpose in a locked post, but sometimes people just click without looking or hit the wrong thing.
Someone wasn't thinking when they decided to roll this out. As usual...