On my best ranking sites, I am writing approximately around 20 up to 30 words for every video. You do not really need anything longer than that and you have to make sure that you are not repeating keywords in every sentence. The rule of thumb is to add your keyword just once or twice in the text so you do not over do it since it can actually lower your search engine ranking positions.
You can take a look at MyLust.com for some samples of how descriptions should look like.
Also remember that your video title is more important and it has to be original.