Here’s the TL;DR: Oven at 350°F. 8lb bone-in: 2–2.5 hours. Boneless: 1.5–2 hours. Internal temp: 160°F. Rest: 20 minutes. That’s it. No PhD needed.
What if it’s done early? Keep it in a warm oven (around 200°F) covered in foil for up to an hour. Just don’t let it dry out. Or, heck, serve it immediately with gravy and mashed potatoes. Who’s judging? Not me.
So, my friend, how long do you cook an 8lb turkey breast? You cook it until it’s done. And now you know exactly how done that is. Go forth. Roast. Eat well. Text me the pictures.