Once in a month, sometimes once in two weeks. I am not sure if search engines like you to change article every day. I tried with some hubs, the reaction of search engines was negative. But maybe this is just my experience. Fresh content is definitely a must if you have a site. Its' part of algorithm. The more content the better. I do not know how they look at article stuff. Wikipedia updates its' articles almost daily. If there is somebody who can add something to this or that article. Hope that helped.