I basically write a new blog posts when I come up for an idea for one. Even sometimes when I start to write a blog posts it can take up to a week before I have the blog post finished and published on my blog.
If you can you should aim to write a good blog post everyday but it's not required.
Although the more content you have on your blog the more content can be indexed by google and can result in more visitors and more chance of ranking for keywords in google search engine.
It's better to have a few quality blog posts though than it is to have 1,0000's of low quality useless content.