Storage is often the most under-planned part of a CCTV system. The number of days you can keep depends on the number of cameras, their resolution, the frame rate, the compression (H.265 saves a lot of space compared with H.264) and whether you record continuously or on motion.
For shops and offices, 14–30 days is a common target, so incidents noticed late can still be reviewed. Banks and some regulated sites may need longer.
What retention do you run, and has it ever saved you?