In my personal experience I built my my boat in 1979, I have a Bukh 2G105 29 HP Danish built engine, I have only put one ring of 1/2" packing in my stuffing box, the total amount of water dripping through my stern gear is about 1 gallon, I have have used plenty of grease as I think grease is cheap, stern gear isn't.
The engine is mounted solidly the shaft moves backwards and forwards through the stuffing box about 1" a time every time I change from forward to reverse.
My boat is available for anyone to inspect at any time, at the moment I am moored on the Northern Coventry Canal