Higher the number of days recording needs higher storage space. In other words, number of days of recording is limited by availability of storage space.
To increase the number of days of recording for a given storage (in other words, to optimize the storage requirement for the N number of days recording), we can depend on the statistics (probability of need of older records).
If the statistics says, 90% of the cases, we need recordings in past 1 week, we should maintain full resolution for the last 1 week (say clear car number plate etc...) and reduce the resolution as the number of days increase (we can only see a car accident(/bank robbery) and can't read the number plate) (the probability of needing them will be low).
The implementation shall provision to stop "aging of video record". So that police/CBI can instruct mall / traffic dept to stop aging as they are investigating a case/accident.