The most important issue is a smooth flow of animation along with a good story line.

As long as the story is good, any length is good.

Personally I'd go for 1 minute 30 seconds and above.

PS
All the best.

Depends. My Intolerance Flashes were like 5 minutes and I did them in less than a week. But I suppose if you have epic skills it would be shorter because it would take more time to do a scene.

I'm gonna assume over two minutes. >.>

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.

If you ask me....

How intense the fight is.... that should affect how long the battle should be.

And of course, how much free time you have, that should also be another factor to affect the length of the battle. XDDD

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.

Well it seems to me that you always put alot of detail in your pics. I think that you shouldn't push yourself so I said 30 sec due to your asome kickass attention to detail

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.

2 minutes, only because i was able to squeeze that much into a flash in 2 weeks >: D
and i know your better at me with time management so you could pull it off *laughs evily*

e__e do your best! Immortal animator!!