How long it takes depends on whether you will be using escutcheon plates over them, or not. If not, I drill three holes from each side inside the rectangle that represents the key, with the stock still "fat" along the forend. I use a thin chisel made from a thin flat file, less than the thickness of the key, to break out the wood between the holes right to the lug, from each side. then I use the same chisel to square up the outward ends of the rectangle, as Rich said. From here, I heat a long "key" that is a little undersized to a dull red, and gently push it through, cleaning out the rest of the wood. Sometimes, if the keys are to be of steel, I use the key itself to do the burning, but if brass, I make one of steel. When you dress the stock forend wood down to finished size, you remove the burnt mess and find that the key is an exact fit.
There should be something in the tutorial section on this, I think.
