When you see that bedding is no longer identifiable, you will want to harvest. Worms cannot survive in their own waste. Now if you are raising worms, you harvest the worms. We harvest the castings. The choice is yours.
Some folks sort the worms out of the castings and put the worms in fresh bedding.
We have other things to do with our time and prefer a split harvest method. It helps if you have trained your worms ahead of time for this harvest method. To train your worms, you start feeding them at only one end of the bin. Do this for about a week. You will lose some worms, but those were the ones that were not very smart.