Seems like a decent compromise would be to record a 2 hour loop but as soon as the words "declaring emergency" or similar are detected it stops looping and just records everything till the storage is full. Say 25 hours worth. The point being that at the end of 25 hours it's no longer recording as there is no chance of anything relevant still being said at that time.

Could even have some indicator in the cockpit that it's in emergency mode for the pilots to turn it back to loop mode in case of a false positive.