The most common recommendation is to backup files at least once a week, daily if practical. Of course, individual needs may vary. Some factors to consider are the importance of the files and how frequently they change. It may be redundant to make new backups every day for files that you know won't change. 

One technique many automated backup solutions use to address this is to make incremental backups. An incremental backup only backs up data that has changed or been added since the previous backup. With this approach, there will usually be a new full backup made relatively infrequently, with incremental backups made on a frequent cadence in between full backups.
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