It appears that comment flags will never expire?
I think that comment flags should also expire, because if a comment hasn't been flagged 6 times within a given period, it no-longer deemed note-worthy, just like questions and answers.
Comment flags are now handled by moderators. And since moderators generally process flags within 24 hours, it seems unnecessary to expire those flags.
Back when comment flags were handled by the community, they expired after something like 4 or 6 days (I don't remember exactly), a far longer period of time than it would take the moderators to process them.