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,

