I've had beers ferment out in 24 hours before, especially if it's warm. Even if it's done with the majority of the fermentation, though, the yeast are still busy finishing up and then they start digesting their own waste products. Keep it at the optimum fermentation range for at least another 5 days and then you can check the SG and see if it's done.
Broken Leg Brewery
Giving beer a leg to stand on since 2006