A quarterly evaluation is reasonable and healthy since you will also be testing your competition. Designing for search engines is a very important topic which we address more on our Site Promotion page. If you don't have content that becomes outdated, you should reconsider your content. A stagnant site rarely gets repeat visitors. Assuming you have changing content, keep it updated and fresh and inline with your other marketing efforts.
Most web sites evolve over time and often restructure their pages.
That's good news. The bad news is that search engines and bookmarks have long memories . As pages become obsolete or restructured, a redirect page of the same name can be installed to at least reroute the visitor to a valid page. It's both courteous and, in the case of business sites, professional.