There are two different types of flags that one can raise:
Moderator-only flags which can only be seen and handled by moderators.
These age-away/expire based on guidelines given in the following question:
Review flags which go to their respective review-queues (low quality, close/duplicate).
Only canonical migration flags expire. Since, Ask Ubuntu doesn't have any canonical migration target (as of now), it is irrelevant for us.
Based on my experience, most of the flags get handled within a couple of hours. However, in case of Review flags it can stretch to a couple of weeks. And, in case of Moderator-only flags which might require some investigation it can stretch to a couple of days.
As to your belief that since the flag is taking time to be handled, you might have raised a bad flag — don't worry about it. If a flag is taking time to be handled, it generally indicates that you have raised a good flag. I would leave its interpretation upto you :-)