If my memory serves me correctly the old rule of thumb when I joined QF on the B747 classic and before this info was provided on the flight plans, was the aircraft would burn the additional fuel (added over and above the flight plan TOW) over a 27 hour flight/period.
For example with a flight time of 9 hours, if you added an additional 3000kg the extra burn would be 1000kg, leaving you with only 2000kg additional at destination. On a 3hr flight an additional 1800kg would have an extra burn of 200kg leaving only 1600kg of the extra 1800kg at the other end.
Based on numbers given by the previous posters it may still appear to be reasonably accurate with the newer aircraft.