Is there a limit on how long a flag (on a question/answer, not on a comment) can wait for a moderator's attention?
Does it vary from site to site (I believe it may take longer on SO)?
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It completely depends on the site and how active/busy the moderators are. It doesn't ever "expire" so they'll get to it eventually. Generally, flagging on Meta sites will also take a bit longer, simply because Meta's atmosphere is more laid back and the flags aren't always as clear-cut (and can be debatable).
Things to keep in mind:
Flags are organized into separate groups depending on the type of the flag. Some moderators will go through some of the smaller, or easier, sections (like too chatty flags) first just to knock off some flags swiftly before moving into larger, or harder, sections (like the dreaded "other" flags).
Flags are sorted by the number of flags on the post (a post with 2 flags on it will always appear above a post with 1 flag on it), then sorted by the date they were cast. So if you're the first flag on a post, your flag is going to be all the way at the end of the queue. Depending on the size, it can take a while.
Stack Overflow in particular usually has a massive amount of active flags.
Recommend Closure flags do not ever go to the moderator queues. They only go to the Close Votes review queue, so they won't be handled until users review the question you flagged. Depending on the size of the queue, this can cause them to linger for a while.
Certain flag types can be reviewed by other users aside from moderators or handled automatically. For example, Very Low Quality and Not an Answer flags can bump posts into the Low Quality Posts review queue, where any user with 2,000 reputation can review it and cause an automated response on your flag, depending on their actions.
Your flag may have already been viewed. If you cast a really complex or debatable flag, it could sit their for a while while moderators determine what they want to do.
Note: Flag weight no longer exists in any form and is no longer used to sort the flags queue.
There is no limit on the time a flag can stay active. If your flag is not handled for a long time (e.g. several days) this usually means that the issue is more complicated and the moderators are intentionally leaving the flag as a reminder. This can happen on issues that need more investigations like sock puppeting or vote fraud, or on divisive issues where the mods might want to wait for more community input or are divided on the issue themselves.
If this happens often even for simple issues, the site might have too few active moderators and might need some more pro-tems or an election.
One of the factors that determine how long your flag has to wait for review in the queue is your own flag weight, ie, the ratio of helpful flags to declined flags .So it depends on the user itself up to an extend.
I too had asked a question somewhat similar to this, but not the same topic,and all that I understood from the reviews is that a decrease in the flag weight can slightly lower the priority of your future flags in the moderator queue. On large sites (The SO/SF/SU trilogy, etc), flag with care, since there are lots more flags and the mods need to use their time efficiently. Flag weight matters here. On smaller sites, it's OK, Flag weight doesn't matter here as there are very few active flags at any point of time.
So, based on these, we can be sure that how long your flag has to wait to get reviewed will depend on:
1.your current flag weight balance
2.and of course,size of the site too.It may vary from site to site.