That really depends on how you manage the bin. It has a lot to do with how and when you harvest.

I just want to add that you can get nearly pure worm castings if you let the bin work for a while(say 6-8 weeks)without adding any bedding or food. I don't usually do that because I want to keep the worms as fat and sassy as possible and the vermicompost that you harvest earlier than that is a good product for the garden without the extra time expended. If you leave the worms to work an unfed un bedded bin, they will turn it into a very fine and friable product, but hey, to me its not a necessary process. To each his or her own and happy worming. steve

You can also supplement with some ground up oats here and there to keep the worms fat. They'll still eat the bedding and remaining food in it as long as you don't overdo it. I also let my bins go to castings, but I feed the worms oats every couple days after I stop adding scraps. That also keeps them from wandering.