Some businesses should blog every day. But only do this if you are a keen writer, and have readers who WANT to hear from you every day.
www.Less-is-more.com.au has a blog called The Daily De-Clutter - a tip day on decluttering your home.
Most should post at least every 2 weeks or so. This lets people know you are still in business, and makes you think about what you are doing that is newsworthy. It gives you a reason to promote yourself to your friends, and keeps your site fresh, relevant and alive.