Question: When a post is flagged for moderator attention, does the flag expire, or is it visible to the moderators whenever they check the list of the flagged posts?  

I know that comment flags age away after 4 days, but is that valid also for "moderator attention" flags?

Answer: 
According to the [blog post](http://blog.stackoverflow.com/2009/04/raising-a-red-flag/), they expire after two days. Of course, that was two years ago, and a lot has changed since then, so this may now be significantly different.

In any case, moderator flags should not be ignored for so long that they expire—they should either be attended to or dismissed by the mods.
Moderator flags do not expire. In fact, even if they are dismissed, they will reappear if someone else flags the same post.