Well... for those without much idea on how much goes into writing...
If you figure up the average page of writing to be about 500 words (which is generally true), and then take the length of a choice story all told... Let's say 250,000 words for sake of argument.
That's five hundred pages of writing. Or, let's go even more simple- a mere 100,000 words. The minimum required for a choice story. 200 pages of writing.
Let's say someone is a fast writer, and averages five pages of writing per day. Said person could blaze through and make a game in 40 days, assuming they could keep up a pace of a blazing fast five pages per day (aka 2500 words per day). Plus, people usually have days where they don't work on writing at all- be it work, or hanging out with friends, or whatever, prevents it.
For myself, I've set myself a schedule that I consider 'strict'. Seven chapters of story plus a floating 'miscillaneous' chapter. Goal of ~250k words, with a development time of one chapter every two months. It's certainly do-able, but still a bit nerve-wracking at 15k words per month. A page a day. There's not a lot of room to fall behind- but if the whole story ends up shorter than the goal, I'm giving myself the out of possibly making it expandable into a multi-parter. It's possible to bust out 100k in a month's time. I've done it before- but it takes up more or less the WHOLE month to do so. A year is probably a good time frame for a contest like this. It does keep some pressure on, but isn't so heavy with the pressure as to feel daunting or off-putting. But anyone who wants to participate is likely to be far better off beginning immediately rather than thinking that there's time to spare.