When can I stop writing content?

If you want to continually improve SEO, then never, you shouldn't ever stop writing content. There are two main aims for adding content; to build up your website as authoritve, knowledgable site on relevant topics and also to keep the site regularly updated to be seen as reliable to the search engines.

When working on an search engine optimisation project we push and push for more content to be added to the website in a themed structure so that the website becomes naturally knowledgable on each topic written about in the content. The more content you have, the more knowledgable the site will become, as long as you keep within the themes and topics and don't start writing irrelevant or spamming information.

If you've reached a point where the site is ranking well in the natural listings and you're happy with the results you're getting, don't then stop writing content or optimising because you need to continue working on SEO to maintain those high results. You could slow down on the content updates to weekly instead of daily, but those updates tell search engines that your site is constantly fresh with new data so stopping those updates would make your site less reliable and results may start to drop over time.

Another ways to keep your site updated without much effort would be to add a forum for users to update.