G'day Gabriel, and Welcome to linux.org
What was used to make these backups?
Where are they stored ?.....same hard drive as the OS?...or an external hard drive ?
Which OS are you running?....which desktop etc
A very good suggestion, but the example you have posted finds and deletes files that were modified more than 7 days ago.Find command is very useful option here. You can search for files older then 1 month and delete them with -delete option.
$ find /tmp -mtime +7 -type f -name '*.tmp' -delete