Pages disappear from catalogs. Links degrade . Watch for trouble, and resubmit if you spot it. The engines have grown smart enough to realize some sites only change content once or twice a year, so they don't visit regularly. Resubmitting every month or two will ensure that your site's content is kept current.
Some search engines take weeks or months before they update their catalogs.
For example, in mid-April 1996, WebCrawler's catalog only listed finds through Feb. 1996. The same seemed true for Alta Vista. On the other hand, Excite's catalog consistently reflected changes soon after they were made, an indication that the catalog is constantly updated. However, recent reports reflect that they too are slowing down considerably due to the massive volume of submissions. For one thing, spamming doesn't seem to work with every search engine.