Is there a chart that helps determine how much DSLR video an SD card will hold?

Page 132 in the 550D operator’s manual linked in the sticky above.

I have a 32 gig card, and the camera says it will hold only around 30 minutes. The chart indicates I should get 40 minutes per 16 gig card. Is there some setting I haven't got right?

FAQ - minutes left always shows 29:59 regardless of how big the card is.

Unless there's less than 29:59, of course.

The minutes left will never display more than 29:59 remaining because that is the recording time limit on these cameras. This 30 minute limit is due to the additional camcorder tax in Europe which applies to cameras which can record more than 30 minutes of continuous footage. Even though it says 29:59 remaining, you can still record up to about 45 minutes on a 16GB card.

There is no precise answer as to how much footage can fit on a card of any given size, because the Canon DSLR's use a variable bitrate compression. If there is lots of detail and movement in the scene, the bitrate will be higher and you'll run out of card space faster. Solid colours and locked off shots will result in lower bitrates and longer record times.

A quick way of doing the math is to use the fact that, under normal circumstances, 4GB of footage is about 12 minutes. Then do the math according to the size of your card and you'll see that you'll get about 20-24 minutes on an 8GB card, about 40-45 minutes on a 16GB card etc.