The answer is "as long as they need to be". There is no magic number.
If you write articles that are longer than they need to be then you will put people off reading them, because they will be full of filler. People are more reluctant to read long articles on the web than in print.
If you write articles that are shorter than they need to be then you will be missing out on important points or explanations, so people are less likely to fully understand them.
Above all, remember that you are writing for people, not for search engines.