A 640x480 still image is approximately 8kb in size. Another way to say this is that 125 still images would take up about 1 megabyte. A 1024x768 image is about 20kb, or 50 images per megabyte.

Assuming you took 100 pictures per day for 30 days at 1024x768, these 3000 images would only need 60 megabytes of storage space. Even with 8 cameras, you could still store one month's images in half a gigabyte (at 100 images per day).