Blogging for search engine traffic requires skill and understanding in some areas, however one of the easier areas to accommodate is 'volume'. Many of our clients are intimidated at the prospect of blogging until they understand that you don't have to write an entire tome for each post; a couple paragraphs will do.
In fact, more often than not, a couple of paragraphs is all you need to get your point across clearly and effectively. No need to write more than is necessary, and in most cases filler will be detrimental rather than helpful.
How long should each post be? As a rule of thumb, 300 words is plenty. There is no specific penalty for writing more, although you may loose your customers after the first few hundred words. The search engines are happy with as little as a few paragraphs. 300 words is a good number because it is short enough to write quickly, and long enough to include some good human-readable content with lots of relevant keywords.
In a pinch, don't be afraid to write a few sentences. The SEO value of the content will, of course, not be as strong, however you will be keeping the search engines on their toes as far as being seen as 'fresh and active'. When it comes down to it, frequency is pretty important. If your choice is between being able to write something now, or having to wait until you have time to produce more; always write now.
By the way, this post has 261 words in it. Just about right.