I was just curious as to how long people believe a "playable" demo for an upcoming game should be released.

I think pre-release marketing is very important for a game's success. A good example is Halo 3 (TBA 2007), where there's all these vidocs and screens being released months ahead of time. A bad example is Farcry (2003 I think), which is absolutely incredible (or so I've heard) but didn't sell well because nobody knew it existed for the first few months (they eventually released a demo and I downloaded it but it wouldn't run).