The bits got to go somewhere. So if you want to record the video feed and use the most lowrate option there is:
8 Mbyte / resolution * bits/pixel * frames/second =
(8*1048576 *

/(320*256 *4 *15) = 13.66 seconds of video
Tell me where else the bits would go? keep in mind that the card seems not to be able to do digital video compression (ie mpeg4 etc). With low resolution and for an Amiga fast harddisc controller that manage 600 kB/s realtime disc saving is one option. That is if realtime DMA from video capture won't collide with disc transfers.
If one uses something else than video capture, like instructions to mix existing video feeds or overlay graphics. The demand for storage is significantly less.