I can't answer for sure but when I plan city surviellance system for big cities I limit the system to keep information for 1 month (for an ordinary client) or 3 months (for a reach client). If there was no event, than after the time period the recording is deleted by another recording.

The format today is usually digital video format. Nobody uses tapes anymore. It is recorded on hard drives. You can use the ordinary AVI type of files or more specific codecs with better compression rate (to save space without losing information).

Omer