I don't use a worm farm as I'm an avid DIY'er, my system is basically the same thing except made from rubbermaid's. Â The yield of castings depends how much you feed them, red wigglers will consume anywhere from 1/2 to their full weight in food daily (i.e 1 lbs of worms eats 1/2-1 lbs per day), so it'd logically follow that they'd produce slightly less castings then that each day.
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My current bin has been running for 3.5-4 months now and I have roughly 3-4 inches of worm castings on the bottom. Â I'm actively going out to collect food for them though, I get a bucket of coffee grounds from a local free trade organic roaster/cafe as well as harvesting invasive weeds to add while I'm hiking. Â The goal being to have enough fertilizer to run a 600w HPS indoor garden through the winter to grow all of our greens, tomatoes, strawberries and a couple other odd veggies in a sustainable system.