maybe you should consider doing some practice runs.
quickly doing the math. if you were to go 1/min, thats 60/hour, and lets say 12 hours of shooting, comes out to 720 frames. If you were going to play back at 30 frames/sec, that comes out to a 24 second movie clip. If you were going 1/5 mins, thats a 4.8 second clip. if you were going with 12/min -> 720/hour -> 8640 over 12 hours and a 288 second movie clip (4min 48 sec).
of course you can change your video play back rate, i.e. have each picture show for more than 1/30 of a second, which would increase your clip time. Also consider, how it will look and what you are emphasizing. for example, if you wanted to see cars spinning around on a podium, you want to make sure that you get enough shots in for each rotation or else it might just look like the cars are just pointing in random directions on playback. If you wanted to emphasize setting up the show, the show itself, and clean up, then a larger interval would work.
either way, I am also concerned you will run into power issues if you are only using batteries. It would be nice if you knew before hand approximately how long/how many shots each one can do do and dont forget that the lcd eat up a lot of power (though I have heard if you plug in the av cable, it turns off the screen). Also consider, if you have to change batteries, it will be difficult to do so and keep the exact same framing, if you were planning to shoot the entire show from one spot.
definitely a good idea to hit "smootube' and 'bimeo" for car show examples and ideas.