The proxy configuration in the EFW web interface does not allow you to change the refresh_pattern settings, you will need to edit the /etc/squid/squid.conf.tmpl file (on Windows use WinSCP)
The squid.conf.tmpl file is used by EFW to create the squid.conf file so do not edit the squid.conf file directly as the changes will be overwritten when the Proxy is restarted.
Here are the default settings in the squid.conf.tmpl file for EFW 2.5.1
# caching settings
refresh_pattern -i (/cgi-bin/|\?) 0 0% 0
refresh_pattern . 0 20% 4320
You will need to change these to your settings and then restart the Proxy
Of course take a backup copy of the original file first
Hope this helps.