Add the following line to your crontab using the command "crontab -e" (or EDITOR=$editor crontab -e):
$ 0 17 * * * /usr/bin/find ~/.cache/* -mtime +7 -exec /usr/bin/rmtrash -rf {} ;
This command looks at files in ~/.cache every day at 5 p.m. and moves files older than 7 days to the trash.
Additionally: This can also be done using regular "rm" instead of "rmtrash".