The amount changes depending on lighting, scene detail, and the amount of movement being recorded, but generally 1080p recorded on a new iPad takes up 180MB per minute. So that would be approximately 1.8 GB per hour. So you are looking at nearly 12GB to record 6 hours, about 10GB for 5 hours. If you don't move the iPad much and there is little action, it will take up less. If there is a lot of action but you never have to move the iPad, it will take a little more. If you are constantly pointing the iPad at different things, it will take a lot more. No matter what you are filming, you should be able to film 6 hours with only 14GB free. So on a 16gb iPad you would probably never be able to film for more than 5 hours unless you have literal not installed or downloaded anything. With a 32gb iPad you'd probaly be able to download a moderate amount of apps/games and podcasts/albums/movies and still record 6 hours of video.
