We left the yarn cooking on low for about 2 hours. One hour probably would have been enough, but I was uncertain whether or not the milky-white water indicated that the yarn had soaked up all the dye. When, after another hour, it was still milky, I decided it was probably fine. I unplugged the crock pot, took off the cover, and we went to the mall.
About four hours later, we came home, and the yarn was cool enough to handle. I dumped it into the colander in the sink and rinsed my new, colorful yarn in water the same temperature before putting it through another spin cycle in the washer. Behold, hand-dyed yarn! The colors are pretty close to the photo, although the red is a little more burgundy (not so pinkish) in real life.