The amount of worm castings will vary depending on your food scrap wastage. If you add 30 litres of food to the Subpod weekly, then it will take approximately 6 months to generate 80 liters/21 US gallons of worm castings.
During the initial setup phase, you will be adding food scraps little by little. Once your Subpod is in full force then your worms will have grown in numbers and they will be producing more and more good stuff (worm castings) for your garden every day!