I've made turkey breasts a couple of times in my crockpot. I really don't remember how long I cooked them for, just long enough so the meat fell apart when I stuck a fork into it, which was probably around 5 hours. I added Au Jues sauce and maybe a half inch of water. They both turned out tender and juicy, in both cases I shredded the turkey for turkey sandwiches after eating a portion of it with rice, with some of the au jus sauce over the top of the rice.

I think I had the crockpot on high for a few hours and then turned it down to medium.