I thought the recorders only record the last half hour. I’m not sure that in the scenario painted they can help pinpoint the cause. The cause of the crash would be running out of fuel...
I think the cockpit voice recorder only records for 30 minutes at a time.
I’d guess that there are many data recorder log files for various systems, with varying capacities before overwriting. The more dire of the date being recorded, expect the capacity to be more extensive.
The voice recorder is required to record at least a half hour but can be longer. The data recorder has a longer requirement. Wikipedia (yes I know the warnings) states “Most FDRs record approximately 17–25 hours worth of data in a continuous loop”.
Unless the pilot was still at the helm at that time and took it down just before it ran out of fuel.
What I still don’t understand is if something catastrophic happened (mechanical, fire, explosion or hijacking) which caused the plane to turn around, why didn’t the plane go down soon after the turn? Could a disabled plane continue to fly for hours on auto? Or if it was a hijacking or terrorism, why not do it sooner for maximum impact and bragger’s rights? Truly puzzling.
Most modern passenger aircraft have been upgraded to two hours on an NTSB "recommendation," but your point is still valid.