@Farhansa, nobody can tell you the exact figure, the gear oil becomes so greasy in cars that run on the same oil for years that its impossible for the whole of oil to come out.
For cars with little miles on them like 20k - 30k , the oil comes out almost fully, still a 25 to 50ml maybe still inside, for that you may use that sucking machine
Why dont you do what i advised you before, buy 2 liters first, or if you want to save buy the 3 liter pack, whats the big deal bhai ?