If you do not need to minimize time and media spent on backups, you can do full backups every day. However, this is not realistic for most sites, so incremental backups are used most often. In this case, you should back up your site enough to restore files from the last four weeks. This requires at least four sets of tapes--one for each week, which you would reuse each month. In addition, you should archive the monthly backups for at least a year, and then keep yearly backups for a number of years.