There is no hard and fast answer. It really depends on:
- how prolific you can be,
- how many articles you are submitting at one time, and
- what you are using this technique to accomplish.
Let me go over each one in a little more detail.
How prolific can you be?
How many articles can you write in a week? 1? 2? 50? How often you submit your articles is very much dependent on how many articles you can write. Obviously if you can only write one a week, you won't be submitting more than once a week.
How many articles can you submit at one time?
I don't think there is any article directory rule that limits how many articles you can submit at one time. That said, if the resource box is pointing to the same website for each of them, I would limit it to two or three. You want to build your back links in a more organic manner and suddenly having 50 back links to your site from one directory may get you dinged for content spam by the search engines.
What are you using article marketing to accomplish?
Are you using this technique to improve off-page SEO through back links? Are you using it to drive traffic to your site? Are you using it to prime your list-building pump? Each of these goals will be facilitated by a slightly different strategy.
So, how often should I submit to article directories?
I know people who submit articles three to five times a week. These folks are usually using article marketing to support their affiliate marketing efforts. In this case, volume can make a big difference.
I believe that once a week to once a month is sufficient for steady, organic traffic growth in most cases. But, of course, the more articles you post to directories the better.
To "win" the article marketing race, quantity is quite important. So, posting more often will help you reach large numbers more quickly.
Ultimately, the question you should be asking is, "How does article marketing fit within my overall business strategy?". Your answer to this question will help you decide how often you should be posting.