How many hours of recordings can a HDD contain?

It depends on the resolution, quality and number of images per second you set (the higher the resolution, quality and frequency, the smaller the number of recording hours the HDD can hold). As a guideline, consider that with a resolution of 320x240, a quality of 70% and 2 images per second, recordings take up about 1MB per minute per channel. But if you set to record with movement detection and optimise the movement detection settings, the number of recording hours can be increased enormously: if there is no movement nothing will be recorded.