this is like asking "how fast does an object fall?" Without knowing the resistance the object has while its falling its impossible to answer.
Likewise, without knowing the type of food in question its impossible to know how fast it is absorbed into the body. Straight glucose for instance is absorbed almost instantaneously. Whole wheat bread dunked in olive oil and smeared with brie is absorbed much, much slower (think hours like 6 or 7 even!).
The optimal time to burn fat is right after lifting intense weights. The weight lifting will mobilize free fatty acids into the bloodstream and the cardiovascular exercise following this will burn that fat for energy so it doesn't redeposit.