What would be the instant result/achievement of switching from bi-weekly to monthly? I doubt that it would automatically increase the number of MH members actually participating in the signature contest.
I've been participating in the bi-weekly signature contest(s) since Feb '10 and I'm very happy with the current setup.
Extending the duration of these contests to one whole month would be a bad move imo. Currently, we have a small group of active members that participate in these contests on a regular basis. The theme choices play a key part in attracting potential participants but it's not the sole cause of "a lack of submissions" in the end. There have been cases in the past where Tsukisama has extended the deadline for an additional week to allow more members to complete their pieces and submit them in; which always works out for the best.
I and a few other members have brought up the issue of a lack of participants before and yes it is a problem. However, I believe there are better methods of resolving the issue as oppose to switching to a complete monthly-setup in all honesty. The only positive I can see atm from such a setup would be, more votes overall.
Here’s some feedback from my fellow peers

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The contest being bi-weekly is perfect.
Why? because we, participants, will get bored of waiting and will eventually forget the contest and quit...
I totally disagree with changing the contest status to monthly because I don't like waiting :S Instead I wouldn't mind if it becomes weekly

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I come here for motivation! Being able to compete with other artists and following a precise theme is something that you cannot do on your own. Furthermore, graphix play an important role in the manga universe, so instead of making a competition each mounth, I suggest that we add even more contests so that people who are interested in this field may find something to do when they finish reading and replying to people's posts in the discussion threads...
That's just my opinion, and I will accept any good decision with solid arguments.
I think bi-weekly is really good... one week for thinking and one for executing XD
Too long dead lines just don`t work with me... even though we would have more time to discuss ideas, I think the pressure of a short dead line is better than the lazyness of having a month to do it... hope you guys understand this @__@
I think the most difficult part of a bi-weekly contest is thinking of the themes, actually XD
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oh, and I kind of had an idea for my sig now XD
I agree with not changing it.I get bored waiting for voting results and for the other theme to be posted

More time would mean more pressure IMO. When you have too many ideas, it can be a bad thing to choose which sig to use.
One theme, one week to submit the piece and the following week is when the voting takes place. It’s a great setup and I think it should continue in that manner. The short time we have to come up with ideas for the theme and actually finish the piece before the deadline actually works out better as oppose to having a whole month to consider several options. The pressure of the one week deadline imo actually inspires/motivates members to work at a decent pace so that they can actually complete their signature before the deadline is up.
I think you’re trying to contrast some of those monthly manga chapter releases to this contest; which doesn’t sound too good. Let me use a personal example: D. Gray Man.
I had a slight interest in the manga during it’s weekly chapter faze; however, I suddenly lost complete interest in the manga once it was switched to a monthly release only. Now, this is not a good example but this is how I see the future of the signature contest if it becomes monthly. There won’t be much appeal to it, voting would take place AFTER the one month? That’s just crazy. Too long imo. I would only consider it for special contest events.
The MH Birthday Header Contest would be great as a one month contest that would obviously only take place once a year. I think you should consider taking part in at least one of the bi-weekly signature contests and see for yourself whether you like it or not. Maybe, it'll trigger some cool ideas that you could put forward to the MH staff and members to consider for the future?
I agree with Shin about coming up with new contests within the Art Section. There's a lot of potential in that section. It's just coming up with new and alternative contests that take a bit of time lol.