How long should I cook an 8.5lb bone in turkey breast?

My, that is large. This guide should help! Just set up the oven times and temp like you would for an 8.5 pound whole bird. https://food52.com/blog/11646-the-essential-numbers-a-guide-to-times-and-temps-for-cooking-your-bird

The oven would work! You'll just want to check the internal temperature of the breast to see when it's ready. Here's a guide to times and temps: https://food52.com/blog/11646-the-essential-numbers-a-guide-to-times-and-temps-for-cooking-your-bird

And, while the recipes uses boneless breast, it seems from the comments that folks have followed it with a bone-in breast. You could also debone the breast and follow this recipe: https://food52.com/recipes/31989-torrisi-s-turkey