I keep thinking that the camera must think that there is little space left on the card.
Time for an experiment:
First - there are so many things you can set on that camera, there may be something set that is causing the problem. It is time to reset the camera to factory default settings.
Go to page 176 and reset the camera to default.
Then check the movie recording length and see if the problem is resolved.
Second - Put the card into a card reader on your computer and check its capacity.
Use Windows Explorer and in the left panel you should find the card listed under "Computer".
Right Click on the card (which may be called "removeable disk").
It should show you a pie chart showing the Used Space and the Free Space.
What do you find there?