You may not be able to get stats over the last couple months.
ifconfig (at the command line) will show you the total bytes sent and received since your last reboot on all network interfaces.
Some of the tools listed on the administration page will give you some limited information:
* sarg will produce http proxy download statistics for each local computer going as far back as your squid logs (/var/log/squid/*)...
* sysmon may show average network throughput over time, and hence might be used to calculate an approximated network traffic total.