If the answer is more than a month ago, that's too long. And if you have an e-commerce site, updating doesn't just mean adding a few products or changing their descriptions. Nor does editing a page a bit count. Updating means substantial additions or editing.
What's the big deal?
Updating your site is important for 2 entities - your human visitors and your robot visitors. Humans like fresh stuff to read or look at and keeps them returning - that's why blogs are great additions. It may also get them to bookmark you or, even better, refer you to friends and collegues. Robot visitors - those are the electronic spiders which search engines send out - also like fresh content, but for a different reason. Fresh content indicates that the site owner cares about their site, and is putting effort into making it better = authoritive.
What should I do then?
Write - pure and simple. However, that's not totally helpful, so let's flesh that out a bit. Here are some easy ways of updating and addign material to your site.
Blog - A blog is the easiest way to keep new material pumping into your site. I recommend weekly, but some clients are daunted by that so I tell them to start off doing one every 2 weeks. You can write about almost anything. The one thing I recommend you NOT doing is self-promotion - let the rest of your site do that. Make the blog, generally, industry relevant and educational. However, throw in some "human interest" stuff too. Last year I blogged about being at my daughter's wedding. That sort of thing is good for showing people you're a real person.
Add new services/products pages - Most site owners never fully develop pages about every aspect of the services and products they sell. So write a page about each. Rather than just saying in your "Services" page that you offer, say, "Graphic design", write a page about the topic and include your specializations, experience, past designs - even some testimonials. It makes the spiders happy, and adds more credibility to your business in human eyes.
Remember that ALL content should be rigorously checked for spelling and grammatical errors. Yes, some still get through, but poorly written, spelling and grammar riddled articles turn people off. Also make sure your content is unique and useful. I'm not saying you would intentionally steal content, just ensure that you're not just a "me too".
