A:AnswerYes you can leave it for 24hrs, but would recommend a plug in hard drive, so that when it heats up, it will not fail like a flash drive over time.
A:AnswerThat STRONGLY depends on how the video is compressed -- in other words, which format. However, as for holding movies, One to two hours per gigabyte is probably a good approximation. So a 16 GB device will hold around 16 to 32 hours.
However, don't depend on a flash drive to be indestructible. If your data matters, store is MULTIPLE places and on multiple drives so that if something happens to one, you have another.