When should I take the worm castings out of the bin? Should I wait until the bin is pretty much completely converted into castings? Or take it out little by little? Thanks!
Wait until the bin is pretty much converted. How long that takes is partially dependent on you. Adding bedding when you feed is always a good idea, but if you do this all the time, you will never reach a point where all of the bedding has been converted. If you stop feeding or adding bedding for a few weeks, most of the remaining bedding will be consumed by the worms, and you can harvest.
To answer that question, it needs to be known where you keep the worms. I keep mine in a shed where the temperatures pretty much follow whatever the outdoor temp is. I do have a small heater supply near my box to get them through the below 30s nights. I have also found that the best way to keep them going is to wait until the Spring months to harvest, because I think the vermicompost acts as an insulator for the worms during the winter. I use my vermicompost in my garden, and don't really start planting until April, so no big rush there. And when the season is finishing about the end of Sept., I let them build their supply again for the coming winter.
I usually harvest my bin from side to side. I'll start feeding only on one side of the bin. The worms slowly migrate to the feeding side. Then, when the other side is pretty much composted and the worms are mostly on the feeding side, I'll harvest the compost on the other side.
Then, I'll take that empty compost space, fill it with leaves, coconut coir, shredded paper and so on, and start migrating the feed over to the new side. The next year or so, I'll harvest from the previous side. That way, I'm always creating new compost and harvesting.
Here is a link that might be useful: How to compost with earthworms