A recorder uses its drives in a loop: when space runs out, it deletes the oldest footage and keeps recording. How many days are kept depends on drive capacity, number of cameras, resolution, compression (H.265) and recording mode.
Decide the retention period up front: incidents are often discovered days or weeks later, by which time the footage may already be gone.
Example
A shop spots a loss at the end of the month — if the archive only holds 7 days, the footage no longer exists.