Baking chicken breast in the oven at 400 F degrees takes from 35 to 45 minutes to be fully cooked.
Before placing the chicken breasts in the oven preheat the oven to 400 F for around 5 to 10 minutes and then place the chicken breasts in the oven and check the chicken breasts after 30 minutes to make sure no blood is in the meat.
If there's still blood in the meat it needs to bake another 15 minutes. If you have a meat thermometer put the meat thermometer in the thickest part of the chicken breast and if the internal temperature is 180 F or higher then the chicken breast is done.