It depends on how drastic you do karate and exercise yourself. If you spend an hour doing some relaxed and not strenuous exercises it doesn't help burn too much calories at all. If you are serious and do some useful movements and training of muscles, it does help you. In fact, doing any kinds of sports other than karate ,can help you burn calories in an hour, such as running, swimming and kung fu. Bear in mind it depends on your program you do.
In a 2 hour class you may actually only do an hour of strenuous karate. I read that in karate you burn 10 calories per minute and for Tai Chi you burn 4 calories per minute. So in one class you can burn anywhere between 480 to 1200 calories. It is realistic that you should look between 600 to 840 for two hours. However, the more you weight the more you burn. A 245 pound 19 year old male can burn 861 per hour while a 145 pound male can burn 626