Your question depends on which type of flag we are talking about. For the flags that are handled by moderators the average handling time is currently less than an hour (the median should be still lower).
There is not really a shortest or longest time in any meaningful sense. Sometimes it will be near instantly sometimes an issue could be kept open for days (this happens mostly if one mod wants others to also see the thing).
However, you are asking about a close flag and those are not dealt with by moderators, but (indirectly) via the close review-queue and close votes.
The reason your flag is still open is that the post is neither:
- voted to close for your reason
- closed (for any reason)
- through review
I don't really know why this one takes relatively long for being reviewed. (Could be the rare tag and the confusedness of the question.)
The reason why no one voted for your reason might be that you actually flagged for a reason other then the one you state now (you flagged 'too broad').
As you see one vote can suffice for marking the flag helpful, so this can go quickly. But the full review process can take longer. Again there are no actual thresholds.
That said, your flag did already have some effect. Namely, it placed the question in the review-queue.