Hi,
As ROYT says its not really normal for the autobox to loose oil,id have a look under the rear of the chassis of your car to see if theres any sign of a fluid leak.You might find there gearbox doesnt drop any oil on the road when parked,but when on the move theres a small fluid leak from the autobox under pressure when its working.Have a look at the exhaust pipe for signs of any fluid leak staining the exhaust.
Is there other problems with the autobox then as you say its due to be replaced.
Chips.