This depends upon several factors. First, the resolution of the video. Higher definition video requires more memory for the same length of recording. Second the number of files stored in the root. Depending upon the operating system used, there can be a finite limit to the number of files allowed in the root (lowest level) folder of the card's memory. If you have many short videos (i.e. small files), they may exhaust the capacity of the root folder's directory entries long before the card runs out of memory. To combat this, create folders in which to store videos. It is better for organization as well.
Hope that this helped.