I don't believe you can put a definite number on the DFDR recording duration. DFDRs use proprietary data compression routines so the actual amount of time before they have to re-record varies depending on what the aircraft is actually doing - if it's sat powered on the ground with nothing much happening except the clock ticking you'll get a LOT more than 25 hours I'd expect. (I'm not familiar with the details of A320 DFDRs, but that's certainly true for some other types, and I'd be shocked if the Airbus units were vastly different)