Everyday! Seriously. If you cannot think of something to say about your business everyday then don't blog.
Okay all joking aside. Do you want the real answer? It really depends on how your business blog fits in to your overall sales/marketing strategy. Now I know what you are thinking. "Oh here he goes again, talking about strategies and plans. All I want to do is write a blog!"
Here is an example. I currently send a weekly email with advice and tips for business owners. In this email there is a link to my blog. If I do not have at least one new blog post when this email goes out then people unsubscribe. How do I know this? Because I measured it. The unsubscribes increased by an average of 12% on the weeks where I had nothing new on my blog.
So I need to write at least one blog post a week. This is bare minimum for me; it may be different for you and your business. But it highlights the point because the blog has a direct impact on my email marketing. It also has a wider impact on my search engine ranking and here is why:
Every blog post you publish gets you more traffic and higher up the search engines.
Hand on heart this is the truth, but it's not like a slot machine where you put in a blog post and win more traffic. It may take two years of writing a blog post every day (730 blog posts), then WHAM!
Your site jumps to #1 on Google, you get 80% more traffic, who as a result of your blog spend loads of time reading articles, which increases each articles position on Google, which results in even more traffic and so on and so on.
Now I have answered your question. It is your turn to answer mine. If you knew this then why wouldn't you blog every hour of every day?!?