What is the roasting time per pound for a spatchcock turkey breast?

The answers duck the question. No, you don't determine doneness by time, only by temperature, but when you're planning a meal, you don't want to have the potatoes done and still have to wait two hours for the turkey. So, you CAN estimate cooking times based on research/experience. I smoke my turkeys so I'm smoking at about 250F. I also spatchcock the birds to even cooking and speed cooking times. But I know I can figure ROUGHLY 15 minutes per pound for whole turkey or breast. If you cook a whole bird or a whole breast not spatchcocked estimate ROUGHLY 30 minutes per pound. If you're roasting at higher temps, cut your estimate to probably 12 minutes per pound spatchcocked, and 20-25 minutes per pound not spatchcocked.

Similar question (with good answer) here.

There is no general formula to relate cooking time for an intact bird to a spatchcocked or butterflied bird. Yes, it will cook faster than a whole turkey. But as some have already said in comments, cooking time "by the pound" is very inaccurate, since cooking time is based on the time it takes heat to penetrate, which will vary depending on shape, volume, location of bones, etc. Weight is not the most relevant measurement. Also, time and appearance of the meat is not enough to determine whether it has reached a hot enough temperature to be safe to eat.

Digital meat thermometers are now available (for only a few dollars in the U.S.) and can help you determine doneness within a matter of seconds. Then you'll be sure your turkey isn't underdone (and unsafe) or overdone (and dry). If you cut your turkey up before cooking, you can even test various parts with the thermometer and remove or foil ones that get done more quickly, so they won't overcook.

If you're willing to pay a little more (probably $15-20 in the U.S.), you can get a digital probe thermometer with a cord to the probe that can stay in the meat while cooking. Most of these even come with an alarm that can be set by temperature, so the moment your bird crosses the threshold of perfect doneness, you will know and can take it out and wrap in foil until dinner is ready. (Also, since the probe remains inside the meat during cooking, you can make adjustments to your oven temperature if it's cooking a little too fast or too slow to be sure the turkey gets done at the right time. This takes a little experience, but is well worth it.)

To me, it's not worth potentially ruining dinner and wasting a large (and often expensive) piece of meat by guessing when it will be done. Cheap thermometers fix that problem, particularly when you're dealing with a new and unfamiliar cooking method.

As for doneness, official USDA standards always say 165F throughout all parts of the bird for safety. Other sources will note that breast meat frequently dries out at that temperature, but I wouldn't recommend going lower than 165F unless you know what you're doing, have read the warnings, and know your method is safe.

I understand the question, since the flat turkey breast cooks quicker than a regular turkey breast. I think it would depend on the size and the degrees of your oven. I'd go to the Butterball site and check out cooking times they recommend and shorten that time, but always use a thermometer to check at the shorten time you decided on. I do this with a 7 lb. breast at 325 degrees for 2 hours and it's done.