FPS is only part of the equation....quality has a minimal impact.
You need to know the codec that is being used for the NVR. Use a MJPEG or MP4V, etc. you're going to use a lot more space than a H264 codec. Also, you need to know what size the image is on top of it....CIF, 2CIF, VGA, etc. That will also affect your storage usage. 15FPS is very reasonable for video.