People seem to like lengthier animations, from my experience with the shorts I've released in the past. (Setting effort/quality aside.) However, there's no set requirement as to how long an animation needs to be. I personally feel that as long as the animation is at least 15-20 seconds, has a nice concept, and shows that you put a good amount of effort into your animation, it'd be acceptable for it to be uploaded. If you try uploading an animation with little/no effort put into the animation, it will most-likely be blamed. At the very least, "Newgrounds" is a site that still values quality and effort. It, along with most of the community, strives to make people be serious about their fields. When someone makes something that they aren't serious about putting any effort into, it shows. For example, I finished an animation commission for someone a few months ago, and they asked me what I thought about what they added. (This was literally just a few hours after I had sent them the part of the animation I was commissioned to do.) So I bluntly told them that if they uploaded it, it would definitely be taken down, since they had simply slapped random, motion tweened images onto what I had animated. :3.
But if you don't want to worry about what others think about the length, quality, etc., you could upload your videos to "YouTube" and make a topic for you to update when you release a new video. Though I'd highly advise not doing so, if you aren't serious about putting the necessary effort into your work. I hope this helps. :3.
