Assuming that's a normal "US" billion, and assuming it's a journal of historical data going back a few years, I don't think it's unreasonable to think there could be information in there on a couple of hundred thousand people each of whom has been track for an average of at least 6 months. So, approximately and with some guesses, that's around 55 records per prisoner per day. 1 update every 30 minutes? That sounds about right, maybe a little on the low side if anything.
What is surprising is that they were running some sort of database process that maxxed out at 2 billion records, and that it just stopped once it hit that limit rather than failing over to a backup process. But then, this is a government IT contract, so maybe it's not too surprising.