The size(s) of your hard drive(s) dictates this. Also effecting this is the number of cameras and recording configuration (on motion, on alarm, continuously) and the video compression in use. If you are recording "on motion" (the most common recording method) using a 30fps system and using 4 CCTV security cameras with motion in the surveilled area for 16 hours a day, 5 days a week, recording at 320x240 pixel resolution with MPEG4 compression, you could expect to use up about 25GB of disk space per week. So, a 200GB drive would hold about about 8 weeks of recordings under those conditions.

If you record at 640x480 pixel resolution, or you are using a 120fps or system, the disk space usage will increase. Because of the method MPEG4 compresses video, there is no specific formula that can be used to indicate how much the recordings space would increase.