Once you have eaten a meal, how long does it take for it to convert into energy for your body?

you need to be more specific, it depends on the food and wat the basic things in it are (carbs fats proteins)

then it depends on the types of carbs fats and proteins(whey, simple, complex, saturated, unsaturated)etc)

then it depends on how fast ur metabolism is and how much fiber u get in ur diet.

etc.

It depends on the food and how much of it you eat. Actually, your saliva starts the digestion process, so I guess you could say instantly... It would depend on how far into it you want to go.