The challenges are there, IMHO, for two reasons
- varying the subjects, by suggesting themes, less asked questions might arise, and the most active users (who usually take part in the challenge) are "forced" to think outside their own issues.
Now having a shorter time for the challenge would have a consequence of a much faster turn around the themes. I can foresee a few consequences
- Faster drying out of themes, or need to often repeat,
- The required thoughts would either keep people very busy, thinking some new questions on a new themes every weeks, or they might get bored and users would participate less.
Either it looses some interest, or it takes precedence over the real purpose of the WB SE, which is to ask questions that you are facing when building your world.
So I'd say that every two weeks is fine. If on the second week there is less participation on the theme, it will leave more time for non-themed questions, and might allow people to ask a follow-up question and still be on the challenge.