I heard it was 350-500 for SEO. But as previously mentioned, as long as it's a "good" "content" article, it doesn't matter how long it is. It would also depend on the topic. Say, review of a product. Should be longer. Describing what you had for dinner. Unless you ate a variety of things, it shouldn't be as long.
250 if you're manually submitting to article directories like ezinearticles 400 if you want more Google SEO juice and you're submitting automatically through article submission services