It will depend on the size of the video and how much memory it takes. If you take a 1 minuet of video and transfer this to your computer you will be able to see how big this file is and work out an aprox time.
From a camera I have that takes videos at 320x240 a 1 min video uses about 7MB from a 512 card it may take 73 min aprox.
Like all cameras they will only be able to short videos and will not be able to fill the card in one take. The more videos you take the quicker you will have to change/recharge your batteries.