@anime roks - If you are talking about how often each manga chapter raw comes out, it varies. In case of FMA, a new raw chapter (in Japanese) comes out each month in Japan.
If you are asking how frequently manga gets updated on MangaViewer site with English scanlated chapters, I can't tell you since I'm not the site manger. Hopefully, the site admin-sama will read this and post the answer. But, as I mentioned, he has been quite busy according to his previous post, so I can't tell you exactly when he'll see your post. Sorry!