There are two different types of flags that one can raise:

1. **Moderator-only flags** which can only be seen and handled by moderators.

 These age-away/expire based on guidelines given in the following question:

 - http://meta.stackoverflow.com/questions/204327/when-are-moderator-flags-automatically-dismissed-as-aged-away-by-the-system

2. **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.

 - http://meta.stackoverflow.com/questions/207179/do-flags-to-close-expire-like-votes-to-close

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 :-)