I was wondering this very question yesterday, and via a search found a response from Clarkson46 saying the gearbox capacity is 3.5 litres but as its nigh impossible to drain it completely you'll only get a little over 3l back in!
As araon says give it time to settle between rounds of topping up, and take the plug out of the topping up hole before draining, just to check you can get it out.