When you are writing or editing your posts and pages, the changes you make are automatically saved every 2 minutes. In the lower right corner of the editor, you’ll see a notification of when the entry was last saved to the database. Autosaves are automatically enabled for all posts and pages. There is only one autosave for each post/page. Each new autosave overwrites the previous autosave in the database.

  • Related articles: Revision Management
  • External links: Wordpress.com Support - Autosave

Reference http://codex.wordpress.org/Glossary