I personally prefered the shorter time span of one month, I think the prizes are great and just fine as is but if you wanted to spread them over more entries that would be ok too.

I can't really speak for others but the Animation challenges are a real turn off for me and might well be for beginners as it's not something you can learn very quickly.

Low poly modeling is another slight turnoff as again it's something that takes time to learn ( learning to model is easier with unlimited polycount but much harder to model the same thing with half the polys).
I am not saying you shouldn't set those challenges ,just a little less frequent I think.

Just my thoughts.

regards J.S.