Hard Drive

The hard drive will fill up based on three variables
The frame rate on the DVR
The resolution setting
The recording rate

You can change the settings on the last two on the DVR

Also make sure motion detection is on as you dont want to record when nothing is happening.

So on a standard system with the correct settings with say 8 cameras and a 250 gig drive it should last 30 days.
As a really rough guide say one camera one gig per day.