Because there are so many different ways of downloading info, chat, browsing, dns etc and they all count towards the volume, I would suggest a kernel module for netfilter (iptables) called quota
You can then use iptables to fine tune the quotas against protocols, destinations, sources etc...
You will need to get this from the patchomatic
download and apply this to your kernel sources and recompile your kernel.
You will have a new QUOTA option to select when you do Make menuconfig.
quotas are set by byte values and are valid for as long as the rule exists, so if you want a daily quota, restart the rules daily etc...