Data retention in MySQL for billing data

We've been running billing for a few years now for about 150 bills. The DB is growing and the bill_data table is responsible for most of the growth. I can't find anything in the Global settings for this and the housekeeping.php script doesn't seem to address this either. 

The billing calculation cronjob was taking more then an hour so we needed to delete old bills and run optimize on the DB to speed this up a bit.

This improvement would be very much appreciated.