20-30k, like Azezel said.
Haegr The Mouintain, on the other hand, would be atleast 3-4 times that number
That is a gross overstatement; A male african elephant consumes 50.000 calories a day. And for such a big creature it has a terrible energy-output. It's warm-blooded AND a herbivore, eating more than 300 punds of grass a day.
As a former competitive swimmer I kept a diet of around 3000-5000 calories a day, with training 30+ hours a week. I'm a 6'1 guy and weighs 170 lbs - didn't really want to go into this.
Even with all the 'superior' genetics, a Space Marine diet of 20k-30k calories a day seems to be appropriate.
However, on a funny note, check out the Elvis daily diet:
The Elvis Diet:
Breakfast (5 pm) - 5,000 calories
six large eggs cooked in butter with extra salt, 1lb of bacon, half a pound of sausages, 12 buttermilk biscuits
Dinner (10p) - 84,000 calories
Two "Fool's Gold" sandwiches [a jar of peanut butter, a jar of strawberry jam, one pound of crisp-fried bacon on a baguette x2]
Supper (4a) - 5,000 calories
5 double-hamburgers and deep-fried peanut butter, mashed banana sandwiches.
Misc. - other snacks as required between meals
Elvis total dietary intake in calories averaged a minimum of 94,000 *per day*. The article highlights that an adult Asian elephant (many tons in weight) has a normal diet of 50,000 calories per day. The article quotes a spokesman for the British Nutrition Foundation as saying, "I do not know how he did it... The Elvis diet would fuel a normal man for a month." The article continues, "Eventually this condition [consuming 94,000 calories worth of food per day] contributed to his death