If you are using the charger that came in the box with the Nexus 10, there is no way that it should be taking 8+ hours to charge the device. 4-5 hours should be all it takes to charge the device from 0% to 100%. If it is taking significantly longer than five hours, either you are using a wrong/defective charger, or your tablet is defective (unlikely).
As others have said, the battery in the Nexus 10 is around five times as large as a normal cell phone battery. Just like it takes a longer time to fill up a gallon of water compared to a quart of water, the larger the container is, the longer amount of time it will take to top it off.