I've gone with 1 oz (by WEIGHT) per gallon of Apfelwein, so 3 oz of Dextrose should work.
If you like it less fizzy, then 2-2.5 should work.
The Tasty Brew calculator shows a wide range of 'standard' volumes of CO2 for cider. Average (2.25 volumes at 65 deg f) comes out to 2.2 oz (by weight)...