CCTV systems use hard drives for recording purposes. The recording hardware has various memory sizes which determine how long the system is able to record for. There are methods to work out how long a certain quality of image can be recorded for.
As a general rule my clientele generally are happy to have anywhere from 2-4 weeks footage on there hard Drives. If you desire to have more footage then depending which hardware you are using more hard drives will need to be added to boost the storage of the CCTV system.
When the recording system is full the recording will overwrite piece by piece until it starts again as a never ending cycle.
The DVR recorders we use currently do have the capability to add more recording by adding the recommended hard drives for extra storage. As technology progresses the size of Hard Drives will increase and so will the capability for DVR`s to have more storage.
IP Systems for larger scale Installations are usually more beneficial due to the fact that the NVR servers are made for much larger Storage operations.