Hi there,
You question is a little ambiguous. Are you looking for the maxium thoughput rate, or the aggregate byte count though the interface?
I have a script that runs over the RRD files and generates the 95th percentil for the last 7 days, this could easily be modified to report the MAX over the last day. I output these to splunk for automated reporting purposes.
If you are after the byte count however, that is a much harder problem. You probably need to turn on some kind of real accounting (radius or netflow) on your device.
Cheers,
Rod.