^^^
Our digital system uses re-recordeable dvds which are swapped out every 30 days. There's half a dozen discs, so at any given time there would 6 months worth of recordings there. But that's not the end of it, the computer that writes on the disc has a hard disc in it that backups up everything. It over writes old info when there is no capacity left rather than after a time. This means the quantity recorded could, and does, go back a couple of years.
I'd imagine this is a pretty standard set up in most units these days.