How often a backup is created is decided by your web host. Contact your hosting administrator for more details.
Important: You should keep your own backup copy of your web site as well. Do not rely solely on the backup provided by your web host. Having multiple backups in different locations provides security against permanently losing information.
Note: A complete web site backup file includes everything - from web pages to images to scripts to access logs. A large site will have a large backup file, and will take some time to download a backup file.