All turkey breast recipes don't belong in a slowcooker. I know it's easier to cook it that way, but c'mon...we can do better than that.
Turkey is as versatile as chicken. There are hundreds of ways to prepare it.
BUT BE CAREFUL!
No-one likes it dry. Too many Thanksgiving turkeys have been ruined because they were overcooked. You also have to be careful that you don't undercook it too. Undercooking may cause nasty bacterias that are still in the meat to make you (and your family) pretty sick.
The turkey breast cooking time depends on size. Also keep in mind that the internal temperature of the meat should reach at least 170 degrees.
Assuming you're cooking in a 325 degree oven,
2-3 pounds cooking time 1 to 2 hours.
3-5 pounds cooking time 1 1/2 to 2 1/2 hours.
6-8 pounds cooking time 2 1/2 to 3 1/2 hours.
It is good idea to have a meat thermometer on hand to measure the internal temperature of your turkey breast. It ensures your meat will be done, and no-one gets sick from eating undercooked food.