OK, I'll be a bit of a contrarian. First off I'm good with deciding on the topics ahead of time and that time period being anything greater than two weeks. I think coming up with a new topic every two weeks is one of the things that bogged the challenge down a bit.
So, just because, I voted other. If we are willing to choose topics in advance (great idea) why not go all the way and choose a years worth. We could build a topic list of say 48 topics and schedule them and post it as a sticky in the forum. Each topic would have the two week period that it will be open already listed beside it. As each topic period opened and closed anyone could check the list and make a new post with approriate boilerplate info (like we do with the weekly contest). The sticky could also serve to archive the links to all the challenges. I chose 48 because we could take two 2 week breaks during the year or use the last 4 weeks of the year to come up with the new topics and take a break. I also feel completely comfortable with "borrowing" other forums challenge or contest topics. The topic is not that important to me (other than being generally accessible to a wide variety of shooters)...it's the challenge of shooting something specific and the comments that are the real value to this.
To sum up:
1 yearly sticky consisting of 48 prescheduled topics
Start each topic on Saturday and end it on Friday 14 days later (we would be on challenge 6 or so now)
Take a 4 week break in Dec to relax and develop the next annual sticky
Heavily borrow ideas from other areas
Any participant can start the new boilerplate challenge and this is key to not letting the challenge fade away
Just a thought, but I'm happy with anything greater than doing the topic every two weeks