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What kind of backup? A Time Machine backup, I presume? Are you talking about a complete backup set from a previous computer or install, or a previous index of your own computer? Are you running out of room on your Time Machine drive?
 
Time Machine keeps hourly backups for the last day, daily backups for the last month, and weekly backups until it fills up your backup drive. It will automatically delete old backups as necessary to free up space for new backups. It will take a very long time to fill up the drive as Time Machine maintains incremental backups, so each subsequent backup is quite small.

Let Time Machine manage its own backups. That's what it's designed to do.
 
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