Depends really, I've seen some amazing action-oriented flashes made in the span of a week that were over 2 minutes long, but that's what I'd expect from an industry professional. At the same time, I've seen some great stuff that was less then a minute but had some awesome buildup and introduction. So I'm going to say that if you work on it for 2 straight weeks, anywhere between 1 and 2 minutes would be acceptable.
I said 1 minute, because the actual fight is rarely what's important, it's the beginning of the flash and end that really have the story progression evident in them, and that's what really counts. Not to mention that these are much shorter rounds, and thusly the animations will probably have to be shorter, and the fights are the things that should really be cut back, not the exposition or ending.
Zeurel, don't strain yourself! From reading what you have posted in your journals I can see making these great flashes come at a cost! I say that making something that would just ruin other things is not always worth it!
Don't get me wrong I enjoy watching these movies along with your comics! Just watching out for your self being.