How much water would one loose if one burns hydrogen and oxygen? By using electrolysis one can separate hydrogen and oxygen, but when burning hydrogen and oxygen together steam is released, therefore creating water. My question is: 


  how much of the original quantity of water would be lost after this
  process?
Theoretically, none.

Actually, since the gases are produced in a damp environment, there would be a lot of water vapor in the gases. You could easily remove most of the water vapor, but some (maybe as much as 1%) would still be in the gases.

I think it would be best to elaborate on what you mean as water being lost?