OpenVPN - Script to keep track of client data usage

I need a script that can keep track of client data usage on openvpn. I already keep track of client usage but only while they are in a session. The problem is that when a client disconnects and reconnects, the data is only calculated during that session and not what the total amount of data used over day, week, month for example. I need a simple script that will run as a cron job, constantly updating a simple one table database showing how much data has been used per hour, per day, week and month. I'd like to see how much data any one client is using at any time. Note that clients always get the same VIP which makes it easier to track. Build it on your own server, share the code until we have this working then I release the payment. I will set up what ever is needed at my end.